Posted on 2008-08-13 16:56
小明 閱讀(3633)
評(píng)論(18) 編輯 收藏 引用 所屬分類(lèi):
C/C++
下面的C++代碼能編譯么？
#include <stdio.h>
#define NUM getnum()
int getnum()
{
int x = 0;
scanf("%d", &x);
printf("%d\n", x);
return x;
}
int main()
{
int array[NUM];
printf("array size =%d\n",sizeof(array));
return 0;
}
在g++中居然可以編譯。
輸入10，返回array size=40
輸入20，返回array size=80
輸入-1，返回array size =-4!!
問(wèn)題：
1.這樣的做法符合C++標(biāo)準(zhǔn)么？連sizeof成了運(yùn)行期計(jì)算
2.這個(gè)array的空間應(yīng)該分配在heap上，而不是stack上。g++做了什么手腳？
答案是：
1. C99的標(biāo)準(zhǔn):
變長(zhǎng)數(shù)組，不過(guò)支持大小為負(fù)數(shù)的數(shù)組就有些奇怪了
Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C89 mode and in
c++. (However, GCC's implementation of variable-length arrays does not yet conform in detail to the ISO C99 standard.) These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited.
2. 其實(shí)是分配在stack上面，動(dòng)態(tài)的調(diào)整esp就可以了