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 taken as 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 or __auto_type (see Referring to a Type with typeof).
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 A is a class, then
A a;
({a;}).Foo ()
constructs a temporary A object to hold the result of the
statement expression, and that is used to invoke Foo.
Therefore the this pointer observed by Foo is not the
address of a.
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,
#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 ());
}
has different places where temporaries are destroyed. For the
macro case, the temporary X is destroyed just after
the initialization of b. In the function case that
temporary is destroyed when the function returns.
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.)
Jumping into a statement expression with goto or using a
switch statement outside the statement expression with a
case or default label inside the statement expression is
not permitted. Jumping into a statement expression with a computed
goto (see Labels as Values) 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,
foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz();
calls foo and bar1 and does not call baz but
may or may not call bar2. If bar2 is called, it is
called after foo and before bar1.