初识typeof

/*
 * min()/max() macros that also do
 * strict type-checking.. See the
 * "unnecessary" pointer comparison.
 */
#define min(x,y) ({ /
 const typeof(x) _x = (x); /
 const typeof(y) _y = (y); /
 (void) (&_x == &_y);  /
 _x < _y ? _x : _y; })

#define max(x,y) ({ /
 const typeof(x) _x = (x); /
 const typeof(y) _y = (y); /
 (void) (&_x == &_y);  /
 _x > _y ? _x : _y; })

 

这是在linux 内核代码kernel里看到的代码：实现较大和较小数的比较，好奇的是其中的typeof，搜索一下才知道：

 

typeof这个东西并不是ISO/IEC 9899:1999里的，也就是说不是标准C的运算符，这是gcc的一个扩展。在gcc的官方文档中单独列了一章来说这个东西（5.6 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 Section 5.39 [Alternate Keywords], page 239.

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:

• 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, let’s 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.