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706 lines
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4 years ago
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<html lang="en">
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<head>
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<title>C++ Dialect Options - Using the GNU Compiler Collection (GCC)</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Invoking-GCC.html#Invoking-GCC" title="Invoking GCC">
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<link rel="prev" href="C-Dialect-Options.html#C-Dialect-Options" title="C Dialect Options">
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<link rel="next" href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options" title="Objective-C and Objective-C++ Dialect Options">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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<style type="text/css"><!--
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="C++-Dialect-Options"></a>
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<a name="C_002b_002b-Dialect-Options"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options">Objective-C and Objective-C++ Dialect Options</a>,
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Previous: <a rel="previous" accesskey="p" href="C-Dialect-Options.html#C-Dialect-Options">C Dialect Options</a>,
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Up: <a rel="up" accesskey="u" href="Invoking-GCC.html#Invoking-GCC">Invoking GCC</a>
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<hr>
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</div>
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<h3 class="section">3.5 Options Controlling C++ Dialect</h3>
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<p><a name="index-compiler-options_002c-C_002b_002b-144"></a><a name="index-C_002b_002b-options_002c-command_002dline-145"></a><a name="index-options_002c-C_002b_002b-146"></a>This section describes the command-line options that are only meaningful
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for C++ programs. You can also use most of the GNU compiler options
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regardless of what language your program is in. For example, you
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might compile a file <samp><span class="file">firstClass.C</span></samp> like this:
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<pre class="smallexample"> g++ -g -frepo -O -c firstClass.C
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</pre>
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<p class="noindent">In this example, only <samp><span class="option">-frepo</span></samp> is an option meant
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only for C++ programs; you can use the other options with any
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language supported by GCC.
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<p>Here is a list of options that are <em>only</em> for compiling C++ programs:
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<dl>
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<dt><code>-fabi-version=</code><var>n</var><dd><a name="index-fabi_002dversion-147"></a>Use version <var>n</var> of the C++ ABI. The default is version 0.
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<p>Version 0 refers to the version conforming most closely to
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the C++ ABI specification. Therefore, the ABI obtained using version 0
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will change in different versions of G++ as ABI bugs are fixed.
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<p>Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
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<p>Version 2 is the version of the C++ ABI that first appeared in G++
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3.4, and was the default through G++ 4.9.
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<p>Version 3 corrects an error in mangling a constant address as a
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template argument.
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<p>Version 4, which first appeared in G++ 4.5, implements a standard
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mangling for vector types.
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<p>Version 5, which first appeared in G++ 4.6, corrects the mangling of
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attribute const/volatile on function pointer types, decltype of a
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plain decl, and use of a function parameter in the declaration of
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another parameter.
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<p>Version 6, which first appeared in G++ 4.7, corrects the promotion
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behavior of C++11 scoped enums and the mangling of template argument
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packs, const/static_cast, prefix ++ and –, and a class scope function
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used as a template argument.
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<p>Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a
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builtin type and corrects the mangling of lambdas in default argument
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scope.
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<p>Version 8, which first appeared in G++ 4.9, corrects the substitution
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behavior of function types with function-cv-qualifiers.
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<p>Version 9, which first appeared in G++ 5.2, corrects the alignment of
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<code>nullptr_t</code>.
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<p>See also <samp><span class="option">-Wabi</span></samp>.
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<br><dt><code>-fabi-compat-version=</code><var>n</var><dd><a name="index-fabi_002dcompat_002dversion-148"></a>On targets that support strong aliases, G++
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works around mangling changes by creating an alias with the correct
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mangled name when defining a symbol with an incorrect mangled name.
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This switch specifies which ABI version to use for the alias.
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<p>With <samp><span class="option">-fabi-version=0</span></samp> (the default), this defaults to 2. If
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another ABI version is explicitly selected, this defaults to 0.
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<p>The compatibility version is also set by <samp><span class="option">-Wabi=</span><var>n</var></samp>.
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<br><dt><code>-fno-access-control</code><dd><a name="index-fno_002daccess_002dcontrol-149"></a>Turn off all access checking. This switch is mainly useful for working
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around bugs in the access control code.
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<br><dt><code>-fcheck-new</code><dd><a name="index-fcheck_002dnew-150"></a>Check that the pointer returned by <code>operator new</code> is non-null
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before attempting to modify the storage allocated. This check is
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normally unnecessary because the C++ standard specifies that
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<code>operator new</code> only returns <code>0</code> if it is declared
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<code>throw()</code>, in which case the compiler always checks the
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return value even without this option. In all other cases, when
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<code>operator new</code> has a non-empty exception specification, memory
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exhaustion is signalled by throwing <code>std::bad_alloc</code>. See also
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‘<samp><span class="samp">new (nothrow)</span></samp>’.
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<br><dt><code>-fconstexpr-depth=</code><var>n</var><dd><a name="index-fconstexpr_002ddepth-151"></a>Set the maximum nested evaluation depth for C++11 constexpr functions
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to <var>n</var>. A limit is needed to detect endless recursion during
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constant expression evaluation. The minimum specified by the standard
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is 512.
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<br><dt><code>-fdeduce-init-list</code><dd><a name="index-fdeduce_002dinit_002dlist-152"></a>Enable deduction of a template type parameter as
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<code>std::initializer_list</code> from a brace-enclosed initializer list, i.e.
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<pre class="smallexample"> template <class T> auto forward(T t) -> decltype (realfn (t))
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{
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return realfn (t);
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}
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void f()
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{
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forward({1,2}); // call forward<std::initializer_list<int>>
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}
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</pre>
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<p>This deduction was implemented as a possible extension to the
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originally proposed semantics for the C++11 standard, but was not part
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of the final standard, so it is disabled by default. This option is
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deprecated, and may be removed in a future version of G++.
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<br><dt><code>-ffriend-injection</code><dd><a name="index-ffriend_002dinjection-153"></a>Inject friend functions into the enclosing namespace, so that they are
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visible outside the scope of the class in which they are declared.
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Friend functions were documented to work this way in the old Annotated
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C++ Reference Manual.
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However, in ISO C++ a friend function that is not declared
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in an enclosing scope can only be found using argument dependent
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lookup. GCC defaults to the standard behavior.
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<p>This option is for compatibility, and may be removed in a future
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release of G++.
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<br><dt><code>-fno-elide-constructors</code><dd><a name="index-fno_002delide_002dconstructors-154"></a>The C++ standard allows an implementation to omit creating a temporary
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that is only used to initialize another object of the same type.
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Specifying this option disables that optimization, and forces G++ to
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call the copy constructor in all cases.
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<br><dt><code>-fno-enforce-eh-specs</code><dd><a name="index-fno_002denforce_002deh_002dspecs-155"></a>Don't generate code to check for violation of exception specifications
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at run time. This option violates the C++ standard, but may be useful
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for reducing code size in production builds, much like defining
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<code>NDEBUG</code>. This does not give user code permission to throw
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exceptions in violation of the exception specifications; the compiler
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still optimizes based on the specifications, so throwing an
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unexpected exception results in undefined behavior at run time.
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<br><dt><code>-fextern-tls-init</code><dt><code>-fno-extern-tls-init</code><dd><a name="index-fextern_002dtls_002dinit-156"></a><a name="index-fno_002dextern_002dtls_002dinit-157"></a>The C++11 and OpenMP standards allow <code>thread_local</code> and
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<code>threadprivate</code> variables to have dynamic (runtime)
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initialization. To support this, any use of such a variable goes
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through a wrapper function that performs any necessary initialization.
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When the use and definition of the variable are in the same
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translation unit, this overhead can be optimized away, but when the
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use is in a different translation unit there is significant overhead
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even if the variable doesn't actually need dynamic initialization. If
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the programmer can be sure that no use of the variable in a
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non-defining TU needs to trigger dynamic initialization (either
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because the variable is statically initialized, or a use of the
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variable in the defining TU will be executed before any uses in
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another TU), they can avoid this overhead with the
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<samp><span class="option">-fno-extern-tls-init</span></samp> option.
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<p>On targets that support symbol aliases, the default is
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<samp><span class="option">-fextern-tls-init</span></samp>. On targets that do not support symbol
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aliases, the default is <samp><span class="option">-fno-extern-tls-init</span></samp>.
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<br><dt><code>-ffor-scope</code><dt><code>-fno-for-scope</code><dd><a name="index-ffor_002dscope-158"></a><a name="index-fno_002dfor_002dscope-159"></a>If <samp><span class="option">-ffor-scope</span></samp> is specified, the scope of variables declared in
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a <i>for-init-statement</i> is limited to the <code>for</code> loop itself,
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as specified by the C++ standard.
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If <samp><span class="option">-fno-for-scope</span></samp> is specified, the scope of variables declared in
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a <i>for-init-statement</i> extends to the end of the enclosing scope,
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as was the case in old versions of G++, and other (traditional)
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implementations of C++.
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<p>If neither flag is given, the default is to follow the standard,
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but to allow and give a warning for old-style code that would
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otherwise be invalid, or have different behavior.
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<br><dt><code>-fno-gnu-keywords</code><dd><a name="index-fno_002dgnu_002dkeywords-160"></a>Do not recognize <code>typeof</code> as a keyword, so that code can use this
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word as an identifier. You can use the keyword <code>__typeof__</code> instead.
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<samp><span class="option">-ansi</span></samp> implies <samp><span class="option">-fno-gnu-keywords</span></samp>.
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<br><dt><code>-fno-implicit-templates</code><dd><a name="index-fno_002dimplicit_002dtemplates-161"></a>Never emit code for non-inline templates that are instantiated
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implicitly (i.e. by use); only emit code for explicit instantiations.
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See <a href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</a>, for more information.
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<br><dt><code>-fno-implicit-inline-templates</code><dd><a name="index-fno_002dimplicit_002dinline_002dtemplates-162"></a>Don't emit code for implicit instantiations of inline templates, either.
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The default is to handle inlines differently so that compiles with and
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without optimization need the same set of explicit instantiations.
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<br><dt><code>-fno-implement-inlines</code><dd><a name="index-fno_002dimplement_002dinlines-163"></a>To save space, do not emit out-of-line copies of inline functions
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controlled by <code>#pragma implementation</code>. This causes linker
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errors if these functions are not inlined everywhere they are called.
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<br><dt><code>-fms-extensions</code><dd><a name="index-fms_002dextensions-164"></a>Disable Wpedantic warnings about constructs used in MFC, such as implicit
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int and getting a pointer to member function via non-standard syntax.
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<br><dt><code>-fno-nonansi-builtins</code><dd><a name="index-fno_002dnonansi_002dbuiltins-165"></a>Disable built-in declarations of functions that are not mandated by
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ANSI/ISO C. These include <code>ffs</code>, <code>alloca</code>, <code>_exit</code>,
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<code>index</code>, <code>bzero</code>, <code>conjf</code>, and other related functions.
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<br><dt><code>-fnothrow-opt</code><dd><a name="index-fnothrow_002dopt-166"></a>Treat a <code>throw()</code> exception specification as if it were a
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<code>noexcept</code> specification to reduce or eliminate the text size
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overhead relative to a function with no exception specification. If
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the function has local variables of types with non-trivial
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destructors, the exception specification actually makes the
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function smaller because the EH cleanups for those variables can be
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optimized away. The semantic effect is that an exception thrown out of
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a function with such an exception specification results in a call
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to <code>terminate</code> rather than <code>unexpected</code>.
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<br><dt><code>-fno-operator-names</code><dd><a name="index-fno_002doperator_002dnames-167"></a>Do not treat the operator name keywords <code>and</code>, <code>bitand</code>,
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<code>bitor</code>, <code>compl</code>, <code>not</code>, <code>or</code> and <code>xor</code> as
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synonyms as keywords.
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<br><dt><code>-fno-optional-diags</code><dd><a name="index-fno_002doptional_002ddiags-168"></a>Disable diagnostics that the standard says a compiler does not need to
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issue. Currently, the only such diagnostic issued by G++ is the one for
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a name having multiple meanings within a class.
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<br><dt><code>-fpermissive</code><dd><a name="index-fpermissive-169"></a>Downgrade some diagnostics about nonconformant code from errors to
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warnings. Thus, using <samp><span class="option">-fpermissive</span></samp> allows some
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nonconforming code to compile.
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<br><dt><code>-fno-pretty-templates</code><dd><a name="index-fno_002dpretty_002dtemplates-170"></a>When an error message refers to a specialization of a function
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template, the compiler normally prints the signature of the
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template followed by the template arguments and any typedefs or
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typenames in the signature (e.g. <code>void f(T) [with T = int]</code>
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rather than <code>void f(int)</code>) so that it's clear which template is
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involved. When an error message refers to a specialization of a class
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template, the compiler omits any template arguments that match
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the default template arguments for that template. If either of these
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behaviors make it harder to understand the error message rather than
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easier, you can use <samp><span class="option">-fno-pretty-templates</span></samp> to disable them.
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<br><dt><code>-frepo</code><dd><a name="index-frepo-171"></a>Enable automatic template instantiation at link time. This option also
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implies <samp><span class="option">-fno-implicit-templates</span></samp>. See <a href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</a>, for more information.
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|
|
|
||
|
|
<br><dt><code>-fno-rtti</code><dd><a name="index-fno_002drtti-172"></a>Disable generation of information about every class with virtual
|
||
|
|
functions for use by the C++ run-time type identification features
|
||
|
|
(<code>dynamic_cast</code> and <code>typeid</code>). If you don't use those parts
|
||
|
|
of the language, you can save some space by using this flag. Note that
|
||
|
|
exception handling uses the same information, but G++ generates it as
|
||
|
|
needed. The <code>dynamic_cast</code> operator can still be used for casts that
|
||
|
|
do not require run-time type information, i.e. casts to <code>void *</code> or to
|
||
|
|
unambiguous base classes.
|
||
|
|
|
||
|
|
<br><dt><code>-fsized-deallocation</code><dd><a name="index-fsized_002ddeallocation-173"></a>Enable the built-in global declarations
|
||
|
|
<pre class="smallexample"> void operator delete (void *, std::size_t) noexcept;
|
||
|
|
void operator delete[] (void *, std::size_t) noexcept;
|
||
|
|
</pre>
|
||
|
|
<p>as introduced in C++14. This is useful for user-defined replacement
|
||
|
|
deallocation functions that, for example, use the size of the object
|
||
|
|
to make deallocation faster. Enabled by default under
|
||
|
|
<samp><span class="option">-std=c++14</span></samp> and above. The flag <samp><span class="option">-Wsized-deallocation</span></samp>
|
||
|
|
warns about places that might want to add a definition.
|
||
|
|
|
||
|
|
<br><dt><code>-fstats</code><dd><a name="index-fstats-174"></a>Emit statistics about front-end processing at the end of the compilation.
|
||
|
|
This information is generally only useful to the G++ development team.
|
||
|
|
|
||
|
|
<br><dt><code>-fstrict-enums</code><dd><a name="index-fstrict_002denums-175"></a>Allow the compiler to optimize using the assumption that a value of
|
||
|
|
enumerated type can only be one of the values of the enumeration (as
|
||
|
|
defined in the C++ standard; basically, a value that can be
|
||
|
|
represented in the minimum number of bits needed to represent all the
|
||
|
|
enumerators). This assumption may not be valid if the program uses a
|
||
|
|
cast to convert an arbitrary integer value to the enumerated type.
|
||
|
|
|
||
|
|
<br><dt><code>-ftemplate-backtrace-limit=</code><var>n</var><dd><a name="index-ftemplate_002dbacktrace_002dlimit-176"></a>Set the maximum number of template instantiation notes for a single
|
||
|
|
warning or error to <var>n</var>. The default value is 10.
|
||
|
|
|
||
|
|
<br><dt><code>-ftemplate-depth=</code><var>n</var><dd><a name="index-ftemplate_002ddepth-177"></a>Set the maximum instantiation depth for template classes to <var>n</var>.
|
||
|
|
A limit on the template instantiation depth is needed to detect
|
||
|
|
endless recursions during template class instantiation. ANSI/ISO C++
|
||
|
|
conforming programs must not rely on a maximum depth greater than 17
|
||
|
|
(changed to 1024 in C++11). The default value is 900, as the compiler
|
||
|
|
can run out of stack space before hitting 1024 in some situations.
|
||
|
|
|
||
|
|
<br><dt><code>-fno-threadsafe-statics</code><dd><a name="index-fno_002dthreadsafe_002dstatics-178"></a>Do not emit the extra code to use the routines specified in the C++
|
||
|
|
ABI for thread-safe initialization of local statics. You can use this
|
||
|
|
option to reduce code size slightly in code that doesn't need to be
|
||
|
|
thread-safe.
|
||
|
|
|
||
|
|
<br><dt><code>-fuse-cxa-atexit</code><dd><a name="index-fuse_002dcxa_002datexit-179"></a>Register destructors for objects with static storage duration with the
|
||
|
|
<code>__cxa_atexit</code> function rather than the <code>atexit</code> function.
|
||
|
|
This option is required for fully standards-compliant handling of static
|
||
|
|
destructors, but only works if your C library supports
|
||
|
|
<code>__cxa_atexit</code>.
|
||
|
|
|
||
|
|
<br><dt><code>-fno-use-cxa-get-exception-ptr</code><dd><a name="index-fno_002duse_002dcxa_002dget_002dexception_002dptr-180"></a>Don't use the <code>__cxa_get_exception_ptr</code> runtime routine. This
|
||
|
|
causes <code>std::uncaught_exception</code> to be incorrect, but is necessary
|
||
|
|
if the runtime routine is not available.
|
||
|
|
|
||
|
|
<br><dt><code>-fvisibility-inlines-hidden</code><dd><a name="index-fvisibility_002dinlines_002dhidden-181"></a>This switch declares that the user does not attempt to compare
|
||
|
|
pointers to inline functions or methods where the addresses of the two functions
|
||
|
|
are taken in different shared objects.
|
||
|
|
|
||
|
|
<p>The effect of this is that GCC may, effectively, mark inline methods with
|
||
|
|
<code>__attribute__ ((visibility ("hidden")))</code> so that they do not
|
||
|
|
appear in the export table of a DSO and do not require a PLT indirection
|
||
|
|
when used within the DSO. Enabling this option can have a dramatic effect
|
||
|
|
on load and link times of a DSO as it massively reduces the size of the
|
||
|
|
dynamic export table when the library makes heavy use of templates.
|
||
|
|
|
||
|
|
<p>The behavior of this switch is not quite the same as marking the
|
||
|
|
methods as hidden directly, because it does not affect static variables
|
||
|
|
local to the function or cause the compiler to deduce that
|
||
|
|
the function is defined in only one shared object.
|
||
|
|
|
||
|
|
<p>You may mark a method as having a visibility explicitly to negate the
|
||
|
|
effect of the switch for that method. For example, if you do want to
|
||
|
|
compare pointers to a particular inline method, you might mark it as
|
||
|
|
having default visibility. Marking the enclosing class with explicit
|
||
|
|
visibility has no effect.
|
||
|
|
|
||
|
|
<p>Explicitly instantiated inline methods are unaffected by this option
|
||
|
|
as their linkage might otherwise cross a shared library boundary.
|
||
|
|
See <a href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</a>.
|
||
|
|
|
||
|
|
<br><dt><code>-fvisibility-ms-compat</code><dd><a name="index-fvisibility_002dms_002dcompat-182"></a>This flag attempts to use visibility settings to make GCC's C++
|
||
|
|
linkage model compatible with that of Microsoft Visual Studio.
|
||
|
|
|
||
|
|
<p>The flag makes these changes to GCC's linkage model:
|
||
|
|
|
||
|
|
<ol type=1 start=1>
|
||
|
|
<li>It sets the default visibility to <code>hidden</code>, like
|
||
|
|
<samp><span class="option">-fvisibility=hidden</span></samp>.
|
||
|
|
|
||
|
|
<li>Types, but not their members, are not hidden by default.
|
||
|
|
|
||
|
|
<li>The One Definition Rule is relaxed for types without explicit
|
||
|
|
visibility specifications that are defined in more than one
|
||
|
|
shared object: those declarations are permitted if they are
|
||
|
|
permitted when this option is not used.
|
||
|
|
</ol>
|
||
|
|
|
||
|
|
<p>In new code it is better to use <samp><span class="option">-fvisibility=hidden</span></samp> and
|
||
|
|
export those classes that are intended to be externally visible.
|
||
|
|
Unfortunately it is possible for code to rely, perhaps accidentally,
|
||
|
|
on the Visual Studio behavior.
|
||
|
|
|
||
|
|
<p>Among the consequences of these changes are that static data members
|
||
|
|
of the same type with the same name but defined in different shared
|
||
|
|
objects are different, so changing one does not change the other;
|
||
|
|
and that pointers to function members defined in different shared
|
||
|
|
objects may not compare equal. When this flag is given, it is a
|
||
|
|
violation of the ODR to define types with the same name differently.
|
||
|
|
|
||
|
|
<br><dt><code>-fvtable-verify=</code><span class="roman">[</span><code>std</code><span class="roman">|</span><code>preinit</code><span class="roman">|</span><code>none</code><span class="roman">]</span><dd><a name="index-fvtable_002dverify-183"></a>Turn on (or off, if using <samp><span class="option">-fvtable-verify=none</span></samp>) the security
|
||
|
|
feature that verifies at run time, for every virtual call, that
|
||
|
|
the vtable pointer through which the call is made is valid for the type of
|
||
|
|
the object, and has not been corrupted or overwritten. If an invalid vtable
|
||
|
|
pointer is detected at run time, an error is reported and execution of the
|
||
|
|
program is immediately halted.
|
||
|
|
|
||
|
|
<p>This option causes run-time data structures to be built at program startup,
|
||
|
|
which are used for verifying the vtable pointers.
|
||
|
|
The options ‘<samp><span class="samp">std</span></samp>’ and ‘<samp><span class="samp">preinit</span></samp>’
|
||
|
|
control the timing of when these data structures are built. In both cases the
|
||
|
|
data structures are built before execution reaches <code>main</code>. Using
|
||
|
|
<samp><span class="option">-fvtable-verify=std</span></samp> causes the data structures to be built after
|
||
|
|
shared libraries have been loaded and initialized.
|
||
|
|
<samp><span class="option">-fvtable-verify=preinit</span></samp> causes them to be built before shared
|
||
|
|
libraries have been loaded and initialized.
|
||
|
|
|
||
|
|
<p>If this option appears multiple times in the command line with different
|
||
|
|
values specified, ‘<samp><span class="samp">none</span></samp>’ takes highest priority over both ‘<samp><span class="samp">std</span></samp>’ and
|
||
|
|
‘<samp><span class="samp">preinit</span></samp>’; ‘<samp><span class="samp">preinit</span></samp>’ takes priority over ‘<samp><span class="samp">std</span></samp>’.
|
||
|
|
|
||
|
|
<br><dt><code>-fvtv-debug</code><dd><a name="index-fvtv_002ddebug-184"></a>When used in conjunction with <samp><span class="option">-fvtable-verify=std</span></samp> or
|
||
|
|
<samp><span class="option">-fvtable-verify=preinit</span></samp>, causes debug versions of the
|
||
|
|
runtime functions for the vtable verification feature to be called.
|
||
|
|
This flag also causes the compiler to log information about which
|
||
|
|
vtable pointers it finds for each class.
|
||
|
|
This information is written to a file named <samp><span class="file">vtv_set_ptr_data.log</span></samp>
|
||
|
|
in the directory named by the environment variable <samp><span class="env">VTV_LOGS_DIR</span></samp>
|
||
|
|
if that is defined or the current working directory otherwise.
|
||
|
|
|
||
|
|
<p>Note: This feature <em>appends</em> data to the log file. If you want a fresh log
|
||
|
|
file, be sure to delete any existing one.
|
||
|
|
|
||
|
|
<br><dt><code>-fvtv-counts</code><dd><a name="index-fvtv_002dcounts-185"></a>This is a debugging flag. When used in conjunction with
|
||
|
|
<samp><span class="option">-fvtable-verify=std</span></samp> or <samp><span class="option">-fvtable-verify=preinit</span></samp>, this
|
||
|
|
causes the compiler to keep track of the total number of virtual calls
|
||
|
|
it encounters and the number of verifications it inserts. It also
|
||
|
|
counts the number of calls to certain run-time library functions
|
||
|
|
that it inserts and logs this information for each compilation unit.
|
||
|
|
The compiler writes this information to a file named
|
||
|
|
<samp><span class="file">vtv_count_data.log</span></samp> in the directory named by the environment
|
||
|
|
variable <samp><span class="env">VTV_LOGS_DIR</span></samp> if that is defined or the current working
|
||
|
|
directory otherwise. It also counts the size of the vtable pointer sets
|
||
|
|
for each class, and writes this information to <samp><span class="file">vtv_class_set_sizes.log</span></samp>
|
||
|
|
in the same directory.
|
||
|
|
|
||
|
|
<p>Note: This feature <em>appends</em> data to the log files. To get fresh log
|
||
|
|
files, be sure to delete any existing ones.
|
||
|
|
|
||
|
|
<br><dt><code>-fno-weak</code><dd><a name="index-fno_002dweak-186"></a>Do not use weak symbol support, even if it is provided by the linker.
|
||
|
|
By default, G++ uses weak symbols if they are available. This
|
||
|
|
option exists only for testing, and should not be used by end-users;
|
||
|
|
it results in inferior code and has no benefits. This option may
|
||
|
|
be removed in a future release of G++.
|
||
|
|
|
||
|
|
<br><dt><code>-nostdinc++</code><dd><a name="index-nostdinc_002b_002b-187"></a>Do not search for header files in the standard directories specific to
|
||
|
|
C++, but do still search the other standard directories. (This option
|
||
|
|
is used when building the C++ library.)
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<p>In addition, these optimization, warning, and code generation options
|
||
|
|
have meanings only for C++ programs:
|
||
|
|
|
||
|
|
<dl>
|
||
|
|
<dt><code>-Wabi </code><span class="roman">(C, Objective-C, C++ and Objective-C++ only)</span><dd><a name="index-Wabi-188"></a><a name="index-Wno_002dabi-189"></a>When an explicit <samp><span class="option">-fabi-version=</span><var>n</var></samp> option is used, causes
|
||
|
|
G++ to warn when it generates code that is probably not compatible with the
|
||
|
|
vendor-neutral C++ ABI. Since G++ now defaults to
|
||
|
|
<samp><span class="option">-fabi-version=0</span></samp>, <samp><span class="option">-Wabi</span></samp> has no effect unless either
|
||
|
|
an older ABI version is selected (with <samp><span class="option">-fabi-version=</span><var>n</var></samp>)
|
||
|
|
or an older compatibility version is selected (with
|
||
|
|
<samp><span class="option">-Wabi=</span><var>n</var></samp> or <samp><span class="option">-fabi-compat-version=</span><var>n</var></samp>).
|
||
|
|
|
||
|
|
<p>Although an effort has been made to warn about
|
||
|
|
all such cases, there are probably some cases that are not warned about,
|
||
|
|
even though G++ is generating incompatible code. There may also be
|
||
|
|
cases where warnings are emitted even though the code that is generated
|
||
|
|
is compatible.
|
||
|
|
|
||
|
|
<p>You should rewrite your code to avoid these warnings if you are
|
||
|
|
concerned about the fact that code generated by G++ may not be binary
|
||
|
|
compatible with code generated by other compilers.
|
||
|
|
|
||
|
|
<p><samp><span class="option">-Wabi</span></samp> can also be used with an explicit version number to
|
||
|
|
warn about compatibility with a particular <samp><span class="option">-fabi-version</span></samp>
|
||
|
|
level, e.g. <samp><span class="option">-Wabi=2</span></samp> to warn about changes relative to
|
||
|
|
<samp><span class="option">-fabi-version=2</span></samp>. Specifying a version number also sets
|
||
|
|
<samp><span class="option">-fabi-compat-version=</span><var>n</var></samp>.
|
||
|
|
|
||
|
|
<p>The known incompatibilities in <samp><span class="option">-fabi-version=2</span></samp> (which was the
|
||
|
|
default from GCC 3.4 to 4.9) include:
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>A template with a non-type template parameter of reference type was
|
||
|
|
mangled incorrectly:
|
||
|
|
<pre class="smallexample"> extern int N;
|
||
|
|
template <int &> struct S {};
|
||
|
|
void n (S<N>) {2}
|
||
|
|
</pre>
|
||
|
|
<p>This was fixed in <samp><span class="option">-fabi-version=3</span></samp>.
|
||
|
|
|
||
|
|
<li>SIMD vector types declared using <code>__attribute ((vector_size))</code> were
|
||
|
|
mangled in a non-standard way that does not allow for overloading of
|
||
|
|
functions taking vectors of different sizes.
|
||
|
|
|
||
|
|
<p>The mangling was changed in <samp><span class="option">-fabi-version=4</span></samp>.
|
||
|
|
|
||
|
|
<li><code>__attribute ((const))</code> and <code>noreturn</code> were mangled as type
|
||
|
|
qualifiers, and <code>decltype</code> of a plain declaration was folded away.
|
||
|
|
|
||
|
|
<p>These mangling issues were fixed in <samp><span class="option">-fabi-version=5</span></samp>.
|
||
|
|
|
||
|
|
<li>Scoped enumerators passed as arguments to a variadic function are
|
||
|
|
promoted like unscoped enumerators, causing <code>va_arg</code> to complain.
|
||
|
|
On most targets this does not actually affect the parameter passing
|
||
|
|
ABI, as there is no way to pass an argument smaller than <code>int</code>.
|
||
|
|
|
||
|
|
<p>Also, the ABI changed the mangling of template argument packs,
|
||
|
|
<code>const_cast</code>, <code>static_cast</code>, prefix increment/decrement, and
|
||
|
|
a class scope function used as a template argument.
|
||
|
|
|
||
|
|
<p>These issues were corrected in <samp><span class="option">-fabi-version=6</span></samp>.
|
||
|
|
|
||
|
|
<li>Lambdas in default argument scope were mangled incorrectly, and the
|
||
|
|
ABI changed the mangling of <code>nullptr_t</code>.
|
||
|
|
|
||
|
|
<p>These issues were corrected in <samp><span class="option">-fabi-version=7</span></samp>.
|
||
|
|
|
||
|
|
<li>When mangling a function type with function-cv-qualifiers, the
|
||
|
|
un-qualified function type was incorrectly treated as a substitution
|
||
|
|
candidate.
|
||
|
|
|
||
|
|
<p>This was fixed in <samp><span class="option">-fabi-version=8</span></samp>, the default for GCC 5.1.
|
||
|
|
|
||
|
|
<li><code>decltype(nullptr)</code> incorrectly had an alignment of 1, leading to
|
||
|
|
unaligned accesses. Note that this did not affect the ABI of a
|
||
|
|
function with a <code>nullptr_t</code> parameter, as parameters have a
|
||
|
|
minimum alignment.
|
||
|
|
|
||
|
|
<p>This was fixed in <samp><span class="option">-fabi-version=9</span></samp>, the default for GCC 5.2.
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<p>It also warns about psABI-related changes. The known psABI changes at this
|
||
|
|
point include:
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>For SysV/x86-64, unions with <code>long double</code> members are
|
||
|
|
passed in memory as specified in psABI. For example:
|
||
|
|
|
||
|
|
<pre class="smallexample"> union U {
|
||
|
|
long double ld;
|
||
|
|
int i;
|
||
|
|
};
|
||
|
|
</pre>
|
||
|
|
<p class="noindent"><code>union U</code> is always passed in memory.
|
||
|
|
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<br><dt><code>-Wabi-tag </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wabi_002dtag-190"></a><a name="index-g_t_002dWabi_002dtag-191"></a>Warn when a type with an ABI tag is used in a context that does not
|
||
|
|
have that ABI tag. See <a href="C_002b_002b-Attributes.html#C_002b_002b-Attributes">C++ Attributes</a> for more information
|
||
|
|
about ABI tags.
|
||
|
|
|
||
|
|
<br><dt><code>-Wctor-dtor-privacy </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wctor_002ddtor_002dprivacy-192"></a><a name="index-Wno_002dctor_002ddtor_002dprivacy-193"></a>Warn when a class seems unusable because all the constructors or
|
||
|
|
destructors in that class are private, and it has neither friends nor
|
||
|
|
public static member functions. Also warn if there are no non-private
|
||
|
|
methods, and there's at least one private member function that isn't
|
||
|
|
a constructor or destructor.
|
||
|
|
|
||
|
|
<br><dt><code>-Wdelete-non-virtual-dtor </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wdelete_002dnon_002dvirtual_002ddtor-194"></a><a name="index-Wno_002ddelete_002dnon_002dvirtual_002ddtor-195"></a>Warn when <code>delete</code> is used to destroy an instance of a class that
|
||
|
|
has virtual functions and non-virtual destructor. It is unsafe to delete
|
||
|
|
an instance of a derived class through a pointer to a base class if the
|
||
|
|
base class does not have a virtual destructor. This warning is enabled
|
||
|
|
by <samp><span class="option">-Wall</span></samp>.
|
||
|
|
|
||
|
|
<br><dt><code>-Wliteral-suffix </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wliteral_002dsuffix-196"></a><a name="index-Wno_002dliteral_002dsuffix-197"></a>Warn when a string or character literal is followed by a ud-suffix which does
|
||
|
|
not begin with an underscore. As a conforming extension, GCC treats such
|
||
|
|
suffixes as separate preprocessing tokens in order to maintain backwards
|
||
|
|
compatibility with code that uses formatting macros from <code><inttypes.h></code>.
|
||
|
|
For example:
|
||
|
|
|
||
|
|
<pre class="smallexample"> #define __STDC_FORMAT_MACROS
|
||
|
|
#include <inttypes.h>
|
||
|
|
#include <stdio.h>
|
||
|
|
|
||
|
|
int main() {
|
||
|
|
int64_t i64 = 123;
|
||
|
|
printf("My int64: %"PRId64"\n", i64);
|
||
|
|
}
|
||
|
|
</pre>
|
||
|
|
<p>In this case, <code>PRId64</code> is treated as a separate preprocessing token.
|
||
|
|
|
||
|
|
<p>This warning is enabled by default.
|
||
|
|
|
||
|
|
<br><dt><code>-Wnarrowing </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wnarrowing-198"></a><a name="index-Wno_002dnarrowing-199"></a>Warn when a narrowing conversion prohibited by C++11 occurs within
|
||
|
|
‘<samp><span class="samp">{ }</span></samp>’, e.g.
|
||
|
|
|
||
|
|
<pre class="smallexample"> int i = { 2.2 }; // error: narrowing from double to int
|
||
|
|
</pre>
|
||
|
|
<p>This flag is included in <samp><span class="option">-Wall</span></samp> and <samp><span class="option">-Wc++11-compat</span></samp>.
|
||
|
|
|
||
|
|
<p>With <samp><span class="option">-std=c++11</span></samp>, <samp><span class="option">-Wno-narrowing</span></samp> suppresses the diagnostic
|
||
|
|
required by the standard. Note that this does not affect the meaning
|
||
|
|
of well-formed code; narrowing conversions are still considered
|
||
|
|
ill-formed in SFINAE context.
|
||
|
|
|
||
|
|
<br><dt><code>-Wnoexcept </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wnoexcept-200"></a><a name="index-Wno_002dnoexcept-201"></a>Warn when a noexcept-expression evaluates to false because of a call
|
||
|
|
to a function that does not have a non-throwing exception
|
||
|
|
specification (i.e. <code>throw()</code> or <code>noexcept</code>) but is known by
|
||
|
|
the compiler to never throw an exception.
|
||
|
|
|
||
|
|
<br><dt><code>-Wnon-virtual-dtor </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wnon_002dvirtual_002ddtor-202"></a><a name="index-Wno_002dnon_002dvirtual_002ddtor-203"></a>Warn when a class has virtual functions and an accessible non-virtual
|
||
|
|
destructor itself or in an accessible polymorphic base class, in which
|
||
|
|
case it is possible but unsafe to delete an instance of a derived
|
||
|
|
class through a pointer to the class itself or base class. This
|
||
|
|
warning is automatically enabled if <samp><span class="option">-Weffc++</span></samp> is specified.
|
||
|
|
|
||
|
|
<br><dt><code>-Wreorder </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wreorder-204"></a><a name="index-Wno_002dreorder-205"></a><a name="index-reordering_002c-warning-206"></a><a name="index-warning-for-reordering-of-member-initializers-207"></a>Warn when the order of member initializers given in the code does not
|
||
|
|
match the order in which they must be executed. For instance:
|
||
|
|
|
||
|
|
<pre class="smallexample"> struct A {
|
||
|
|
int i;
|
||
|
|
int j;
|
||
|
|
A(): j (0), i (1) { }
|
||
|
|
};
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">The compiler rearranges the member initializers for <code>i</code>
|
||
|
|
and <code>j</code> to match the declaration order of the members, emitting
|
||
|
|
a warning to that effect. This warning is enabled by <samp><span class="option">-Wall</span></samp>.
|
||
|
|
|
||
|
|
<br><dt><code>-fext-numeric-literals </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-fext_002dnumeric_002dliterals-208"></a><a name="index-fno_002dext_002dnumeric_002dliterals-209"></a>Accept imaginary, fixed-point, or machine-defined
|
||
|
|
literal number suffixes as GNU extensions.
|
||
|
|
When this option is turned off these suffixes are treated
|
||
|
|
as C++11 user-defined literal numeric suffixes.
|
||
|
|
This is on by default for all pre-C++11 dialects and all GNU dialects:
|
||
|
|
<samp><span class="option">-std=c++98</span></samp>, <samp><span class="option">-std=gnu++98</span></samp>, <samp><span class="option">-std=gnu++11</span></samp>,
|
||
|
|
<samp><span class="option">-std=gnu++14</span></samp>.
|
||
|
|
This option is off by default
|
||
|
|
for ISO C++11 onwards (<samp><span class="option">-std=c++11</span></samp>, ...).
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<p>The following <samp><span class="option">-W...</span></samp> options are not affected by <samp><span class="option">-Wall</span></samp>.
|
||
|
|
|
||
|
|
<dl>
|
||
|
|
<dt><code>-Weffc++ </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Weffc_002b_002b-210"></a><a name="index-Wno_002deffc_002b_002b-211"></a>Warn about violations of the following style guidelines from Scott Meyers'
|
||
|
|
<cite>Effective C++</cite> series of books:
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>Define a copy constructor and an assignment operator for classes
|
||
|
|
with dynamically-allocated memory.
|
||
|
|
|
||
|
|
<li>Prefer initialization to assignment in constructors.
|
||
|
|
|
||
|
|
<li>Have <code>operator=</code> return a reference to <code>*this</code>.
|
||
|
|
|
||
|
|
<li>Don't try to return a reference when you must return an object.
|
||
|
|
|
||
|
|
<li>Distinguish between prefix and postfix forms of increment and
|
||
|
|
decrement operators.
|
||
|
|
|
||
|
|
<li>Never overload <code>&&</code>, <code>||</code>, or <code>,</code>.
|
||
|
|
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<p>This option also enables <samp><span class="option">-Wnon-virtual-dtor</span></samp>, which is also
|
||
|
|
one of the effective C++ recommendations. However, the check is
|
||
|
|
extended to warn about the lack of virtual destructor in accessible
|
||
|
|
non-polymorphic bases classes too.
|
||
|
|
|
||
|
|
<p>When selecting this option, be aware that the standard library
|
||
|
|
headers do not obey all of these guidelines; use ‘<samp><span class="samp">grep -v</span></samp>’
|
||
|
|
to filter out those warnings.
|
||
|
|
|
||
|
|
<br><dt><code>-Wstrict-null-sentinel </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wstrict_002dnull_002dsentinel-212"></a><a name="index-Wno_002dstrict_002dnull_002dsentinel-213"></a>Warn about the use of an uncasted <code>NULL</code> as sentinel. When
|
||
|
|
compiling only with GCC this is a valid sentinel, as <code>NULL</code> is defined
|
||
|
|
to <code>__null</code>. Although it is a null pointer constant rather than a
|
||
|
|
null pointer, it is guaranteed to be of the same size as a pointer.
|
||
|
|
But this use is not portable across different compilers.
|
||
|
|
|
||
|
|
<br><dt><code>-Wno-non-template-friend </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wno_002dnon_002dtemplate_002dfriend-214"></a><a name="index-Wnon_002dtemplate_002dfriend-215"></a>Disable warnings when non-templatized friend functions are declared
|
||
|
|
within a template. Since the advent of explicit template specification
|
||
|
|
support in G++, if the name of the friend is an unqualified-id (i.e.,
|
||
|
|
‘<samp><span class="samp">friend foo(int)</span></samp>’), the C++ language specification demands that the
|
||
|
|
friend declare or define an ordinary, nontemplate function. (Section
|
||
|
|
14.5.3). Before G++ implemented explicit specification, unqualified-ids
|
||
|
|
could be interpreted as a particular specialization of a templatized
|
||
|
|
function. Because this non-conforming behavior is no longer the default
|
||
|
|
behavior for G++, <samp><span class="option">-Wnon-template-friend</span></samp> allows the compiler to
|
||
|
|
check existing code for potential trouble spots and is on by default.
|
||
|
|
This new compiler behavior can be turned off with
|
||
|
|
<samp><span class="option">-Wno-non-template-friend</span></samp>, which keeps the conformant compiler code
|
||
|
|
but disables the helpful warning.
|
||
|
|
|
||
|
|
<br><dt><code>-Wold-style-cast </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wold_002dstyle_002dcast-216"></a><a name="index-Wno_002dold_002dstyle_002dcast-217"></a>Warn if an old-style (C-style) cast to a non-void type is used within
|
||
|
|
a C++ program. The new-style casts (<code>dynamic_cast</code>,
|
||
|
|
<code>static_cast</code>, <code>reinterpret_cast</code>, and <code>const_cast</code>) are
|
||
|
|
less vulnerable to unintended effects and much easier to search for.
|
||
|
|
|
||
|
|
<br><dt><code>-Woverloaded-virtual </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Woverloaded_002dvirtual-218"></a><a name="index-Wno_002doverloaded_002dvirtual-219"></a><a name="index-overloaded-virtual-function_002c-warning-220"></a><a name="index-warning-for-overloaded-virtual-function-221"></a>Warn when a function declaration hides virtual functions from a
|
||
|
|
base class. For example, in:
|
||
|
|
|
||
|
|
<pre class="smallexample"> struct A {
|
||
|
|
virtual void f();
|
||
|
|
};
|
||
|
|
|
||
|
|
struct B: public A {
|
||
|
|
void f(int);
|
||
|
|
};
|
||
|
|
</pre>
|
||
|
|
<p>the <code>A</code> class version of <code>f</code> is hidden in <code>B</code>, and code
|
||
|
|
like:
|
||
|
|
|
||
|
|
<pre class="smallexample"> B* b;
|
||
|
|
b->f();
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">fails to compile.
|
||
|
|
|
||
|
|
<br><dt><code>-Wno-pmf-conversions </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wno_002dpmf_002dconversions-222"></a><a name="index-Wpmf_002dconversions-223"></a>Disable the diagnostic for converting a bound pointer to member function
|
||
|
|
to a plain pointer.
|
||
|
|
|
||
|
|
<br><dt><code>-Wsign-promo </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wsign_002dpromo-224"></a><a name="index-Wno_002dsign_002dpromo-225"></a>Warn when overload resolution chooses a promotion from unsigned or
|
||
|
|
enumerated type to a signed type, over a conversion to an unsigned type of
|
||
|
|
the same size. Previous versions of G++ tried to preserve
|
||
|
|
unsignedness, but the standard mandates the current behavior.
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
</body></html>
|
||
|
|
|