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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 could be
used. For example, you can use it in a declaration, in a cast, or inside
of sizeof
or typeof
.
typeof
is often useful in conjunction with the
statements-within-expressions feature. Here is how the two together can
be used to define a safe “maximum” macro that 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
.
Compatibility Note: In addition to typeof
, GCC 2 supported
a more limited extension which 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 will crash; 3.2.1 and later give an error). Code which
relies on it should be rewritten to use typeof
:
typedef typeof(expr) T;
This will work with all versions of GCC.