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| 22 | <title>Statement Exprs (GNU C Language Manual)</title>
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Statements.html" rel="up" title="Statements">
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| 33 | <link href="Variables.html" rel="next" title="Variables">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Statement-Exprs"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Labels-as-Values.html" accesskey="p" rel="prev">Labels as Values</a>, Up: <a href="Statements.html" accesskey="u" rel="up">Statements</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Statements-and-Declarations-in-Expressions"></span><h3 class="section">19.15 Statements and Declarations in Expressions</h3>
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| 64 | <span id="index-statements-inside-expressions"></span>
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| 65 | <span id="index-declarations-inside-expressions"></span>
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| 66 | <span id="index-expressions-containing-statements"></span>
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| 67 |
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| 68 | <p>A block enclosed in parentheses can be used as an expression in GNU
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| 69 | C. This provides a way to use local variables, loops and switches within
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| 70 | an expression. We call it a <em>statement expression</em>.
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| 71 | </p>
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| 72 | <p>Recall that a block is a sequence of statements
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| 73 | surrounded by braces. In this construct, parentheses go around the
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| 74 | braces. For example:
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| 75 | </p>
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| 76 | <div class="example">
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| 77 | <pre class="example">({ int y = foo (); int z;
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| 78 | if (y > 0) z = y;
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| 79 | else z = - y;
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| 80 | z; })
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>is a valid (though slightly more complex than necessary) expression
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| 84 | for the absolute value of <code>foo ()</code>.
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| 85 | </p>
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| 86 | <p>The last statement in the block should be an expression statement; an
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| 87 | expression followed by a semicolon, that is. The value of this
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| 88 | expression serves as the value of statement expression. If the last
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| 89 | statement is anything else, the statement expression’s value is
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| 90 | <code>void</code>.
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| 91 | </p>
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| 92 | <p>This feature is mainly useful in making macro definitions compute each
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| 93 | operand exactly once. See <a href="Macros-and-Auto-Type.html">Macros and Auto Type</a>.
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| 94 | </p>
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| 95 | <p>Statement expressions are not allowed in expressions that must be
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| 96 | constant, such as the value for an enumerator, the width of a
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| 97 | bit-field, or the initial value of a static variable.
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| 98 | </p>
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| 99 | <p>Jumping into a statement expression—with <code>goto</code>, or using a
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| 100 | <code>switch</code> statement outside the statement expression—is an
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| 101 | error. With a computed <code>goto</code> (see <a href="Labels-as-Values.html">Labels as Values</a>), the
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| 102 | compiler can’t detect the error, but it still won’t work.
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| 103 | </p>
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| 104 | <p>Jumping out of a statement expression is permitted, but since
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| 105 | subexpressions in C are not computed in a strict order, it is
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| 106 | unpredictable which other subexpressions will have been computed by
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| 107 | then. For example,
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| 108 | </p>
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| 109 | <div class="example">
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| 110 | <pre class="example"> foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz();
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| 111 | </pre></div>
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| 112 |
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| 113 | <p>calls <code>foo</code> and <code>bar1</code> before it jumps, and never
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| 114 | calls <code>baz</code>, but may or may not call <code>bar2</code>. If <code>bar2</code>
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| 115 | does get called, that occurs after <code>foo</code> and before <code>bar1</code>.
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| 116 | </p>
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| 117 | <hr>
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| 118 | <div class="header">
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| 119 | <p>
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| 120 | Previous: <a href="Labels-as-Values.html" accesskey="p" rel="prev">Labels as Values</a>, Up: <a href="Statements.html" accesskey="u" rel="up">Statements</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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