These extensions are available in C and Objective-C. Most of them are
also available in C++. Chapter 7 Extensions to the C++ Language, for extensions that apply only to C++.
Some features that are in ISO C99 but not C89 or C++ are also, as
extensions, accepted by GCC in C89 mode and in C++.
6.1. Statements and Declarations in Expressions
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 (Section 6.6 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
will construct a temporary A object to hold the result of the
statement expression, and that will be used to invoke Foo.
Therefore the this pointer observed by Foo will not be the
address of a.
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,
#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 ());
} |
will have different places where temporaries are destroyed. For the
macro case, the temporary X will be destroyed just after
the initialization of b. In the function case that
temporary will be 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-expression that lead to precisely this
bug.)
