摘自GNU C中的typeof

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6.6 Referring to a Type with  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:

• This declares y with the type of what x points to.

            typeof (*x) y;

• This declares y as an array of such values.

            typeof (*x) y[4];

• This declares 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:

• Each argument to the macro appears only once in the expansion of the macro. This prevents the size of the macro expansion growing exponentially when calls to such macros are nested inside arguments of such macros.
• If the argument to the macro has variably modified type, it is evaluated only once when using __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.