sizeof(long(long))這事啥子意思？

zheguzai

sizeof(long(long))這事啥子意思？
求出結(jié)果為1
long(long)怎么解釋？強(qiáng)制轉(zhuǎn)換么？

starwing83

long(long)是函數(shù)類型。

函數(shù)類型的大小是實(shí)現(xiàn)定義的（還是未定義？還是別的什么？）反正意思就是取不到，所以給你返回一個(gè)1意思一下。

koolcoy

沒記錯(cuò)的話，implementation defined的行為在不同的實(shí)現(xiàn)里行為可能不一樣，但是在同一個(gè)實(shí)現(xiàn)里行為必須一樣，不過具體啥行為由實(shí)現(xiàn)決定，可以當(dāng)作實(shí)現(xiàn)的擴(kuò)展來用。

myworkstation

回復(fù) 1# zheguzai


    這本來是個(gè)錯(cuò)誤的語(yǔ)法，是標(biāo)準(zhǔn)所不允許的，但是gcc的擴(kuò)展允許此用法。

c標(biāo)準(zhǔn)中對(duì)sizeof有如下規(guī)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.

根據(jù)上述規(guī)定定function type是不被允許的。

gcc的有如下所述的擴(kuò)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

可見之所以返回1是由于gcc的擴(kuò)展影響的結(jié)果。

w_anthony

這個(gè)long(long)沒見過，說是函數(shù)類型，應(yīng)該怎么理解，貌似沒有遇到它的應(yīng)用場(chǎng)景。
long(*)(long)倒是容易理解，是個(gè)函數(shù)指針，sizeof應(yīng)該是4吧，與這個(gè)long(long)應(yīng)該不同。

myworkstation

回復(fù) 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’’.

根據(jù)標(biāo)準(zhǔn)的解釋，一個(gè)函數(shù)類型描述了一個(gè)函數(shù)及其返回值類型，它的特點(diǎn)是具有返回值和參數(shù)。由此可知long(long)實(shí)際上是一個(gè)Type Name,而這個(gè)Type Name指向function type。

captivated

回復(fù) 6# myworkstation


    好吧， 語(yǔ)言律師。

cobras

好像C99支持函數(shù)類型。以前的不支持吧！

myworkstation

回復(fù) 8# cobras


    支持的，C89就有明確的規(guī)定了。