typeof
Another way to refer to the type of an expression is with typeof
. The syntax of using of this keyword looks like sizeof
, but the construct acts semantically like a type name defined with typedef
.
There are two ways of writing the argument to typeof
: with an expression or with a type. Here is an example with an expression:
typeof (x[0](1))
This assumes that x
is an array of pointers to functions; the type described is that of the values of the functions.
Here is an example with a typename as the argument:
typeof (int *)
Here the type described is that of pointers to int
.
If you are writing a header file that must work when included in ISO C programs, write __typeof__
instead of typeof
. See Alternate Keywords.
A typeof
construct can be used anywhere a typedef name can be used. For example, you can use it in a declaration, in a cast, or inside of sizeof
or typeof
.
The operand of typeof
is evaluated for its side effects if and only if it is an expression of variably modified type or the name of such a type.
typeof
is often useful in conjunction with statement expressions (see Statement Exprs). Here is how the two together can be used to define a safe “maximum” macro which operates on any arithmetic type and evaluates each of its arguments exactly once:
#define max(a,b) \ ({ typeof (a) _a = (a); \ typeof (b) _b = (b); \ _a > _b ? _a : _b; })
The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for a
and b
. Eventually we hope to design a new form of declaration syntax that allows you to declare variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts.
Some more examples of the use of typeof
:
y
with the type of what x
points to. typeof (*x) y;
y
as an array of such values. typeof (*x) y[4];
y
as an array of pointers to characters: typeof (typeof (char *)[4]) y;
It is equivalent to the following traditional C declaration:
char *y[4];
To see the meaning of the declaration using typeof
, and why it might be a useful way to write, rewrite it with these macros:
#define pointer(T) typeof(T *) #define array(T, N) typeof(T [N])
Now the declaration can be rewritten this way:
array (pointer (char), 4) y;
Thus, array (pointer (char), 4)
is the type of arrays of 4 pointers to char
.
In GNU C, but not GNU C++, you may also declare the type of a variable as __auto_type
. In that case, the declaration must declare only one variable, whose declarator must just be an identifier, the declaration must be initialized, and the type of the variable is determined by the initializer; the name of the variable is not in scope until after the initializer. (In C++, you should use C++11 auto
for this purpose.) Using __auto_type
, the “maximum” macro above could be written as:
#define max(a,b) \ ({ __auto_type _a = (a); \ __auto_type _b = (b); \ _a > _b ? _a : _b; })
Using __auto_type
instead of typeof
has two advantages:
__auto_type
, but twice if typeof
is used. Compatibility Note: In addition to typeof
, GCC 2 supported a more limited extension that permitted one to write
typedef T = expr;
with the effect of declaring T to have the type of the expression expr. This extension does not work with GCC 3 (versions between 3.0 and 3.2 crash; 3.2.1 and later give an error). Code that relies on it should be rewritten to use typeof
:
typedef typeof(expr) T;
This works with all versions of GCC.
© Free Software Foundation
Licensed under the GNU Free Documentation License, Version 1.3.
https://gcc.gnu.org/onlinedocs/gcc-4.9.3/gcc/Typeof.html