有同学写了这样的程式: int x; cin >> x; int a[x]; // ... 使用 a[x]... 结果没有问题且执行正确. 理论上 int a[x] 的 size 要是 constant 然後 compiler 才能在 compile time 决定 sizeof(a). (还记得 sizeof 是 compile time operator). 那这里是怎麽回事呢? 我做了一个实验: ============================================== int main() { int x; cin >> x; int a[x]; cout << "sizeof(a) = " << sizeof(a) << endl; cout << "&a = " << &a << endl; cout << " a = " << a << endl; cout << "&a[0] = " << &a[0] << endl; cout << "&a[1] = " << &a[1] << endl; cout << "&a[9] = " << &a[9] << endl; int b[10]; cout << "sizeof(b) = " << sizeof(b) << endl; cout << "&b = " << &b << endl; cout << " b = " << b << endl; cout << "&b[0] = " << &b[0] << endl; cout << "&b[1] = " << &b[1] << endl; cout << "&b[9] = " << &b[9] << endl; int c[10]; cout << "&c = " << &c << endl; cout << " c = " << c << endl; cout << "&c[0] = " << &c[0] << endl; cout << "&c[1] = " << &c[1] << endl; cout << "&c[9] = " << &c[9] << endl; } ============================================= 结果输出是 (on Linux): 10 sizeof(a) = 40 &a = 0xfef7e770 a = 0xfef7e770 &a[0] = 0xfef7e770 &a[1] = 0xfef7e774 &a[9] = 0xfef7e794 sizeof(b) = 40 &b = 0xfef7e7f0 b = 0xfef7e7f0 &b[0] = 0xfef7e7f0 &b[1] = 0xfef7e7f4 &b[9] = 0xfef7e814 &c = 0xfef7e7c0 c = 0xfef7e7c0 &c[0] = 0xfef7e7c0 &c[1] = 0xfef7e7c4 &c[9] = 0xfef7e7e4 We can see: 1. sizeof(a) = 40!! 所以显然 sizeof(a) 并不是 compile time 就决定的, 因为 x 是 cin 才指定为 10 的. 查了许多网页都是一面倒的说 sizeof is a compile time operator... 只有一个 IBM Mac OS X/XL C++ compiler 说: "At compile time, the compiler analyzes the expression to determine its type, but does not evaluate it...." Anyway, it's a mystery... 2. a 的 memory address 比 b and c 小!! Since stack memory gives away its memory in the reverse addressing order, that means 程式先给了 b[10] and c[10] 的 memory space 才给 a[10]. 也就是说, 程式进入 main() 之後就先将 local varible b[10], c[10] 的 memory 准备好, 然後执行到 a[x] 才去决定要用多少 memory 来存 a. =================================== 喔, 对了, 对 int a[x] 如果 x is not a constant 是不能用 initializer list 来 initialize a[x] 的值 (e.g. int a[x] = { 0 } ). 因为 sizeof({0}) 还是有可能比 sizeof(a[x]) 还要大... --

※ 发信站: 批踢踢实业坊(ptt.cc) ◆ From: 59.121.132.121 ※ 编辑: ric2k1 来自: 59.121.132.121 (12/05 22:28)