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.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression
for the absolute value of foo ().
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 void, and thus
effectively no value.)
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:
#define max(a,b) ((a) > (b) ? (a) : (b))
But this definition computes either a or b twice, with bad
results if the operand has side effects. In GNU C, if you know the
type of the operands (here let's assume int), you can define
the macro safely as follows:
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
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.
If you don't know the type of the operand, you can still do this, but you
must use typeof (see Typeof).
Statement expressions are not supported fully in G++, and their fate there is unclear. (It is possible that they will become fully supported at some point, or that they will be deprecated, or that the bugs that are present will continue to exist indefinitely.) Presently, statement expressions do not work well as default arguments.
In addition, there are semantic issues with statement-expressions in C++. If you try to use statement-expressions instead of inline functions in C++, you may be surprised at the way object destruction is handled. For example:
#define foo(a) ({int b = (a); b + 3; })
does not work the same way as:
inline int foo(int a) { int b = a; return b + 3; }
In particular, if the expression passed into foo involves the
creation of temporaries, the destructors for those temporaries will be
run earlier in the case of the macro than in the case of the function.
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.)