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| <title>Using the GNU Compiler Collection (GCC): Statement Exprs</title> |
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| <a name="Statement-Exprs"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="Local-Labels.html#Local-Labels" accesskey="n" rel="next">Local Labels</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Statements-and-Declarations-in-Expressions"></a> |
| <h3 class="section">6.1 Statements and Declarations in Expressions</h3> |
| <a name="index-statements-inside-expressions"></a> |
| <a name="index-declarations-inside-expressions"></a> |
| <a name="index-expressions-containing-statements"></a> |
| <a name="index-macros_002c-statements-in-expressions"></a> |
| |
| <p>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> |
| <p>Recall that a compound statement is a sequence of statements surrounded |
| by braces; in this construct, parentheses go around the braces. For |
| example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">({ int y = foo (); int z; |
| if (y > 0) z = y; |
| else z = - y; |
| z; }) |
| </pre></div> |
| |
| <p>is a valid (though slightly more complex than necessary) expression |
| for the absolute value of <code>foo ()</code>. |
| </p> |
| <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> |
| <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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define max(a,b) ((a) > (b) ? (a) : (b)) |
| </pre></div> |
| |
| <a name="index-side-effects_002c-macro-argument"></a> |
| <p>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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define maxint(a,b) \ |
| ({int _a = (a), _b = (b); _a > _b ? _a : _b; }) |
| </pre></div> |
| |
| <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> |
| <p>If you don’t know the type of the operand, you can still do this, but you |
| must use <code>typeof</code> or <code>__auto_type</code> (see <a href="Typeof.html#Typeof">Typeof</a>). |
| </p> |
| <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 |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> A a; |
| |
| ({a;}).Foo () |
| </pre></div> |
| |
| <p>constructs a temporary <code>A</code> object to hold the result of the |
| statement expression, and that is used to invoke <code>Foo</code>. |
| Therefore the <code>this</code> pointer observed by <code>Foo</code> is not the |
| address of <code>a</code>. |
| </p> |
| <p>In a statement expression, any temporaries created within a statement |
| are destroyed at that statement’s end. This makes statement |
| expressions inside macros slightly different from function calls. In |
| the latter case temporaries introduced during argument evaluation are |
| destroyed at the end of the statement that includes the function |
| call. In the statement expression case they are destroyed during |
| the statement expression. For instance, |
| </p> |
| <div class="smallexample"> |
| <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></div> |
| |
| <p>has different places where temporaries are destroyed. For the |
| <code>macro</code> case, the temporary <code>X</code> is destroyed just after |
| the initialization of <code>b</code>. In the <code>function</code> case that |
| temporary is destroyed when the function returns. |
| </p> |
| <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 expressions that lead to precisely this |
| bug.) |
| </p> |
| <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>) has 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, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz(); |
| </pre></div> |
| |
| <p>calls <code>foo</code> and <code>bar1</code> and does not call <code>baz</code> but |
| may or may not call <code>bar2</code>. If <code>bar2</code> is called, it is |
| called after <code>foo</code> and before <code>bar1</code>. |
| </p> |
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