Posted on 2008-08-13 16:56
小明 閱讀(3643)
評論(18) 編輯 收藏 引用 所屬分類:
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!!
問題：
1.這樣的做法符合C++標準么？連sizeof成了運行期計算
2.這個array的空間應該分配在heap上，而不是stack上。g++做了什么手腳？
答案是：
1. C99的標準:
變長數組，不過支持大小為負數的數組就有些奇怪了
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. 其實是分配在stack上面，動態的調整esp就可以了