The typeof operator returns the type of its argument, which can be an expression or a type. The language feature provides a way to derive the type from an expression. The alternate spelling of the keyword, __typeof__, is recommended. Given an expression e, __typeof__(e) can be used anywhere a type name is needed, for example in a declaration or in a cast.
The typeof operator is an orthogonal language extension provided for handling programs developed with GNU C. The language feature has been extended for the Mac OS X platform to accept a vector type as its operand, when AltiVec language extensions have been enabled.
A typeof construct is of the form:
>>-+-__typeof__-+--(--+-expr------+--)------------------------->< '-typeof-----' '-type-name-'
A typeof construct itself is not an expression, but the name of a type. A typeof construct behaves like a type name defined using typedef, although the syntax resembles that of sizeof.
The following examples illustrate its basic syntax. For an expression e:
int e; __typeof__(e + 1) j; /* the same as declaring int j; */ e = (__typeof__(e)) f; /* the same as casting e = (int) f; */
Using a typeof construct is equivalent to declaring a typedef name. Given
int T[2]; int i[2];
you can write
__typeof__(i) a; /* all three constructs have the same meaning */ __typeof__(int[2]) a; __typeof__(T) a;
The behavior of the code is as if you had declared int a[2];.
For a bit field, typeof represents the underlying type of the bit field. For example, int m:2;, the typeof(m) is int. Since the bit field property is not reserved, n in typeof(m) n; is the same as int n, but not int n:2.
The typeof operator can be nested inside sizeof and itself. The following declarations of arr as an array of pointers to int are equivalent:
int *arr[10]; /* traditional C declaration */ __typeof__(__typeof__ (int *)[10]) a; /* equivalent declaration */
The typeof operator can be useful in macro definitions where expression e is a parameter. For example,
#define SWAP(a,b) { __typeof__(a) temp; temp = a; a = b; b = temp; }
Related References