| <html lang="en"> |
| <head> |
| <title>Statement Exprs - Using the GNU Compiler Collection (GCC)</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Using the GNU Compiler Collection (GCC)"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> |
| <link rel="next" href="Local-Labels.html#Local-Labels" title="Local Labels"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
| 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, |
| 2008 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.2 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Funding Free Software'', the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| ``GNU Free Documentation License''. |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development.--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Statement-Exprs"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Local-Labels.html#Local-Labels">Local Labels</a>, |
| Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">6.1 Statements and Declarations in Expressions</h3> |
| |
| <p><a name="index-statements-inside-expressions-2180"></a><a name="index-declarations-inside-expressions-2181"></a><a name="index-expressions-containing-statements-2182"></a><a name="index-macros_002c-statements-in-expressions-2183"></a> |
| <!-- the above section title wrapped and causes an underfull hbox.. i --> |
| <!-- changed it from "within" to "in". -mew 4feb93 --> |
| A compound statement enclosed in parentheses may appear as an expression |
| in GNU C. This allows you to use loops, switches, and local variables |
| within an expression. |
| |
| <p>Recall that a compound statement is a sequence of statements surrounded |
| by braces; in this construct, parentheses go around the braces. For |
| example: |
| |
| <pre class="smallexample"> ({ int y = foo (); int z; |
| if (y > 0) z = y; |
| else z = - y; |
| z; }) |
| </pre> |
| <p class="noindent">is a valid (though slightly more complex than necessary) expression |
| for the absolute value of <code>foo ()</code>. |
| |
| <p>The last thing in the compound statement should be an expression |
| followed by a semicolon; the value of this subexpression serves as the |
| value of the entire construct. (If you use some other kind of statement |
| last within the braces, the construct has type <code>void</code>, and thus |
| effectively no value.) |
| |
| <p>This feature is especially useful in making macro definitions “safe” (so |
| that they evaluate each operand exactly once). For example, the |
| “maximum” function is commonly defined as a macro in standard C as |
| follows: |
| |
| <pre class="smallexample"> #define max(a,b) ((a) > (b) ? (a) : (b)) |
| </pre> |
| <p class="noindent"><a name="index-side-effects_002c-macro-argument-2184"></a>But this definition computes either <var>a</var> or <var>b</var> twice, with bad |
| results if the operand has side effects. In GNU C, if you know the |
| type of the operands (here taken as <code>int</code>), you can define |
| the macro safely as follows: |
| |
| <pre class="smallexample"> #define maxint(a,b) \ |
| ({int _a = (a), _b = (b); _a > _b ? _a : _b; }) |
| </pre> |
| <p>Embedded statements are not allowed in constant expressions, such as |
| the value of an enumeration constant, the width of a bit-field, or |
| the initial value of a static variable. |
| |
| <p>If you don't know the type of the operand, you can still do this, but you |
| must use <code>typeof</code> (see <a href="Typeof.html#Typeof">Typeof</a>). |
| |
| <p>In G++, the result value of a statement expression undergoes array and |
| function pointer decay, and is returned by value to the enclosing |
| expression. For instance, if <code>A</code> is a class, then |
| |
| <pre class="smallexample"> A a; |
| |
| ({a;}).Foo () |
| </pre> |
| <p class="noindent">will construct a temporary <code>A</code> object to hold the result of the |
| statement expression, and that will be used to invoke <code>Foo</code>. |
| Therefore the <code>this</code> pointer observed by <code>Foo</code> will not be the |
| address of <code>a</code>. |
| |
| <p>Any temporaries created within a statement within a statement expression |
| will be destroyed at the statement's end. This makes statement |
| expressions inside macros slightly different from function calls. In |
| the latter case temporaries introduced during argument evaluation will |
| be destroyed at the end of the statement that includes the function |
| call. In the statement expression case they will be destroyed during |
| the statement expression. For instance, |
| |
| <pre class="smallexample"> #define macro(a) ({__typeof__(a) b = (a); b + 3; }) |
| template<typename T> T function(T a) { T b = a; return b + 3; } |
| |
| void foo () |
| { |
| macro (X ()); |
| function (X ()); |
| } |
| </pre> |
| <p class="noindent">will have different places where temporaries are destroyed. For the |
| <code>macro</code> case, the temporary <code>X</code> will be destroyed just after |
| the initialization of <code>b</code>. In the <code>function</code> case that |
| temporary will be destroyed when the function returns. |
| |
| <p>These considerations mean that it is probably a bad idea to use |
| statement-expressions of this form in header files that are designed to |
| work with C++. (Note that some versions of the GNU C Library contained |
| header files using statement-expression that lead to precisely this |
| bug.) |
| |
| <p>Jumping into a statement expression with <code>goto</code> or using a |
| <code>switch</code> statement outside the statement expression with a |
| <code>case</code> or <code>default</code> label inside the statement expression is |
| not permitted. Jumping into a statement expression with a computed |
| <code>goto</code> (see <a href="Labels-as-Values.html#Labels-as-Values">Labels as Values</a>) yields undefined behavior. |
| Jumping out of a statement expression is permitted, but if the |
| statement expression is part of a larger expression then it is |
| unspecified which other subexpressions of that expression have been |
| evaluated except where the language definition requires certain |
| subexpressions to be evaluated before or after the statement |
| expression. In any case, as with a function call the evaluation of a |
| statement expression is not interleaved with the evaluation of other |
| parts of the containing expression. For example, |
| |
| <pre class="smallexample"> foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz(); |
| </pre> |
| <p class="noindent">will call <code>foo</code> and <code>bar1</code> and will not call <code>baz</code> but |
| may or may not call <code>bar2</code>. If <code>bar2</code> is called, it will be |
| called after <code>foo</code> and before <code>bar1</code> |
| |
| </body></html> |
| |
