sizeof(long(long))这事啥子意思？

zheguzai

sizeof(long(long))这事啥子意思？
求出结果为1
long(long)怎么解释？强制转换么？

starwing83

long(long)是函数类型。

函数类型的大小是实现定义的（还是未定义？还是别的什么？）反正意思就是取不到，所以给你返回一个1意思一下。

koolcoy

没记错的话，implementation defined的行为在不同的实现里行为可能不一样，但是在同一个实现里行为必须一样，不过具体啥行为由实现决定，可以当作实现的扩展来用。

myworkstation

回复 1# zheguzai


    这本来是个错误的语法，是标准所不允许的，但是gcc的扩展允许此用法。

c标准中对sizeof有如下规定：6.5.3.4
The sizeof operator shall not be applied to an expression that has function type or an incomplete type, to the parenthesized name of such a type, or to an expression that designates a bit-field member.

根据上述规定定function type是不被允许的。

gcc的有如下所述的扩展:
5.17 Arithmetic on void- and Function-Pointers

In GNU C, addition and subtraction operations are supported on pointers to void and on pointers to functions. This is done by treating the size of a void or of a function as 1.

A consequence of this is that sizeof is also allowed on void and on function types, and returns 1.

The option -Wpointer-arith requests a warning if these extensions are used.
https://gcc.gnu.org/onlinedocs/gcc-4.0.4/gcc/Pointer-Arith.html#Pointer-Arith

可见之所以返回1是由于gcc的扩展影响的结果。

w_anthony

这个long(long)没见过，说是函数类型，应该怎么理解，貌似没有遇到它的应用场景。
long(*)(long)倒是容易理解，是个函数指针，sizeof应该是4吧，与这个long(long)应该不同。

myworkstation

回复 5# w_anthony

A function type describes a function with specified return type. A function type is characterized by its return type and the number and types of its parameters. A function type is said to be derived from its return type, and if its return type is T , the function type is sometimes called ‘‘function returning T’’. The construction of a function type from a return type is called ‘‘function type derivation’’.

根据标准的解释，一个函数类型描述了一个函数及其返回值类型，它的特点是具有返回值和参数。由此可知long(long)实际上是一个Type Name,而这个Type Name指向function type。

captivated

回复 6# myworkstation


    好吧， 语言律师。

cobras

好像C99支持函数类型。以前的不支持吧！

myworkstation

回复 8# cobras


    支持的，C89就有明确的规定了。