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165 lines
7.6 KiB
HTML
165 lines
7.6 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Statement Exprs - 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="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="next" href="Local-Labels.html#Local-Labels" title="Local Labels">
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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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</head>
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<body>
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<div class="node">
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<a name="Statement-Exprs"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Local-Labels.html#Local-Labels">Local Labels</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.1 Statements and Declarations in Expressions</h3>
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<p><a name="index-statements-inside-expressions-2868"></a><a name="index-declarations-inside-expressions-2869"></a><a name="index-expressions-containing-statements-2870"></a><a name="index-macros_002c-statements-in-expressions-2871"></a>
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<!-- the above section title wrapped and causes an underfull hbox.. i -->
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<!-- changed it from "within" to "in". -mew 4feb93 -->
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A compound statement enclosed in parentheses may appear as an expression
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in GNU C. This allows you to use loops, switches, and local variables
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within an expression.
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<p>Recall that a compound statement is a sequence of statements surrounded
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by braces; in this construct, parentheses go around the braces. For
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example:
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<pre class="smallexample"> ({ int y = foo (); int z;
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if (y > 0) z = y;
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else z = - y;
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z; })
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</pre>
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<p class="noindent">is a valid (though slightly more complex than necessary) expression
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for the absolute value of <code>foo ()</code>.
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<p>The last thing in the compound statement should be an expression
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followed by a semicolon; the value of this subexpression serves as the
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value of the entire construct. (If you use some other kind of statement
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last within the braces, the construct has type <code>void</code>, and thus
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effectively no value.)
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<p>This feature is especially useful in making macro definitions “safe” (so
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that they evaluate each operand exactly once). For example, the
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“maximum” function is commonly defined as a macro in standard C as
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follows:
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<pre class="smallexample"> #define max(a,b) ((a) > (b) ? (a) : (b))
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</pre>
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<p class="noindent"><a name="index-side-effects_002c-macro-argument-2872"></a>But this definition computes either <var>a</var> or <var>b</var> twice, with bad
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results if the operand has side effects. In GNU C, if you know the
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type of the operands (here taken as <code>int</code>), you can define
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the macro safely as follows:
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<pre class="smallexample"> #define maxint(a,b) \
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({int _a = (a), _b = (b); _a > _b ? _a : _b; })
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</pre>
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<p>Embedded statements are not allowed in constant expressions, such as
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the value of an enumeration constant, the width of a bit-field, or
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the initial value of a static variable.
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<p>If you don't know the type of the operand, you can still do this, but you
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must use <code>typeof</code> or <code>__auto_type</code> (see <a href="Typeof.html#Typeof">Typeof</a>).
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<p>In G++, the result value of a statement expression undergoes array and
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function pointer decay, and is returned by value to the enclosing
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expression. For instance, if <code>A</code> is a class, then
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<pre class="smallexample"> A a;
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({a;}).Foo ()
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</pre>
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<p class="noindent">constructs a temporary <code>A</code> object to hold the result of the
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statement expression, and that is used to invoke <code>Foo</code>.
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Therefore the <code>this</code> pointer observed by <code>Foo</code> is not the
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address of <code>a</code>.
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<p>In a statement expression, any temporaries created within a statement
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are destroyed at that statement's end. This makes statement
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expressions inside macros slightly different from function calls. In
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the latter case temporaries introduced during argument evaluation are
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destroyed at the end of the statement that includes the function
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call. In the statement expression case they are destroyed during
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the statement expression. For instance,
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<pre class="smallexample"> #define macro(a) ({__typeof__(a) b = (a); b + 3; })
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template<typename T> T function(T a) { T b = a; return b + 3; }
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void foo ()
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{
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macro (X ());
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function (X ());
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}
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</pre>
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<p class="noindent">has different places where temporaries are destroyed. For the
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<code>macro</code> case, the temporary <code>X</code> is destroyed just after
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the initialization of <code>b</code>. In the <code>function</code> case that
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temporary is destroyed when the function returns.
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<p>These considerations mean that it is probably a bad idea to use
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statement expressions of this form in header files that are designed to
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work with C++. (Note that some versions of the GNU C Library contained
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header files using statement expressions that lead to precisely this
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bug.)
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<p>Jumping into a statement expression with <code>goto</code> or using a
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<code>switch</code> statement outside the statement expression with a
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<code>case</code> or <code>default</code> label inside the statement expression is
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not permitted. Jumping into a statement expression with a computed
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<code>goto</code> (see <a href="Labels-as-Values.html#Labels-as-Values">Labels as Values</a>) has undefined behavior.
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Jumping out of a statement expression is permitted, but if the
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statement expression is part of a larger expression then it is
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unspecified which other subexpressions of that expression have been
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evaluated except where the language definition requires certain
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subexpressions to be evaluated before or after the statement
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expression. In any case, as with a function call, the evaluation of a
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statement expression is not interleaved with the evaluation of other
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parts of the containing expression. For example,
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<pre class="smallexample"> foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz();
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</pre>
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<p class="noindent">calls <code>foo</code> and <code>bar1</code> and does not call <code>baz</code> but
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may or may not call <code>bar2</code>. If <code>bar2</code> is called, it is
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called after <code>foo</code> and before <code>bar1</code>.
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</body></html>
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