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173 lines
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173 lines
8.2 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Name lookup - 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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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" title="C++ Misunderstandings">
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<link rel="prev" href="Static-Definitions.html#Static-Definitions" title="Static Definitions">
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<link rel="next" href="Temporaries.html#Temporaries" title="Temporaries">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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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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funds for GNU development.-->
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</head>
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<body>
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<div class="node">
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<a name="Name-lookup"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Temporaries.html#Temporaries">Temporaries</a>,
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Previous: <a rel="previous" accesskey="p" href="Static-Definitions.html#Static-Definitions">Static Definitions</a>,
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Up: <a rel="up" accesskey="u" href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings">C++ Misunderstandings</a>
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<hr>
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</div>
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<h4 class="subsection">12.7.2 Name Lookup, Templates, and Accessing Members of Base Classes</h4>
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<p><a name="index-base-class-members-4282"></a><a name="index-two_002dstage-name-lookup-4283"></a><a name="index-dependent-name-lookup-4284"></a>
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The C++ standard prescribes that all names that are not dependent on
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template parameters are bound to their present definitions when parsing
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a template function or class.<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a> Only names that are dependent are looked up at the point
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of instantiation. For example, consider
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<pre class="smallexample"> void foo(double);
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struct A {
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template <typename T>
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void f () {
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foo (1); // <span class="roman">1</span>
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int i = N; // <span class="roman">2</span>
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T t;
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t.bar(); // <span class="roman">3</span>
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foo (t); // <span class="roman">4</span>
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}
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static const int N;
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};
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</pre>
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<p>Here, the names <code>foo</code> and <code>N</code> appear in a context that does
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not depend on the type of <code>T</code>. The compiler will thus require that
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they are defined in the context of use in the template, not only before
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the point of instantiation, and will here use <code>::foo(double)</code> and
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<code>A::N</code>, respectively. In particular, it will convert the integer
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value to a <code>double</code> when passing it to <code>::foo(double)</code>.
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<p>Conversely, <code>bar</code> and the call to <code>foo</code> in the fourth marked
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line are used in contexts that do depend on the type of <code>T</code>, so
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they are only looked up at the point of instantiation, and you can
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provide declarations for them after declaring the template, but before
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instantiating it. In particular, if you instantiate <code>A::f<int></code>,
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the last line will call an overloaded <code>::foo(int)</code> if one was
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provided, even if after the declaration of <code>struct A</code>.
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<p>This distinction between lookup of dependent and non-dependent names is
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called two-stage (or dependent) name lookup. G++ implements it
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since version 3.4.
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<p>Two-stage name lookup sometimes leads to situations with behavior
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different from non-template codes. The most common is probably this:
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<pre class="smallexample"> template <typename T> struct Base {
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int i;
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};
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template <typename T> struct Derived : public Base<T> {
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int get_i() { return i; }
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};
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</pre>
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<p>In <code>get_i()</code>, <code>i</code> is not used in a dependent context, so the
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compiler will look for a name declared at the enclosing namespace scope
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(which is the global scope here). It will not look into the base class,
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since that is dependent and you may declare specializations of
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<code>Base</code> even after declaring <code>Derived</code>, so the compiler can't
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really know what <code>i</code> would refer to. If there is no global
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variable <code>i</code>, then you will get an error message.
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<p>In order to make it clear that you want the member of the base class,
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you need to defer lookup until instantiation time, at which the base
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class is known. For this, you need to access <code>i</code> in a dependent
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context, by either using <code>this->i</code> (remember that <code>this</code> is of
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type <code>Derived<T>*</code>, so is obviously dependent), or using
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<code>Base<T>::i</code>. Alternatively, <code>Base<T>::i</code> might be brought
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into scope by a <code>using</code>-declaration.
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<p>Another, similar example involves calling member functions of a base
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class:
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<pre class="smallexample"> template <typename T> struct Base {
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int f();
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};
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template <typename T> struct Derived : Base<T> {
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int g() { return f(); };
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};
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</pre>
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<p>Again, the call to <code>f()</code> is not dependent on template arguments
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(there are no arguments that depend on the type <code>T</code>, and it is also
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not otherwise specified that the call should be in a dependent context).
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Thus a global declaration of such a function must be available, since
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the one in the base class is not visible until instantiation time. The
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compiler will consequently produce the following error message:
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<pre class="smallexample"> x.cc: In member function `int Derived<T>::g()':
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x.cc:6: error: there are no arguments to `f' that depend on a template
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parameter, so a declaration of `f' must be available
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x.cc:6: error: (if you use `-fpermissive', G++ will accept your code, but
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allowing the use of an undeclared name is deprecated)
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</pre>
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<p>To make the code valid either use <code>this->f()</code>, or
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<code>Base<T>::f()</code>. Using the <samp><span class="option">-fpermissive</span></samp> flag will also let
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the compiler accept the code, by marking all function calls for which no
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declaration is visible at the time of definition of the template for
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later lookup at instantiation time, as if it were a dependent call.
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We do not recommend using <samp><span class="option">-fpermissive</span></samp> to work around invalid
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code, and it will also only catch cases where functions in base classes
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are called, not where variables in base classes are used (as in the
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example above).
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<p>Note that some compilers (including G++ versions prior to 3.4) get these
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examples wrong and accept above code without an error. Those compilers
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do not implement two-stage name lookup correctly.
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> The C++ standard just uses the
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term “dependent” for names that depend on the type or value of
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template parameters. This shorter term will also be used in the rest of
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this section.</p>
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<hr></div>
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</body></html>
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