C++ 静态数组与动态数组类似

Question

似乎记得 C++ 中的静态数组只能从 const 表达式初始化，但如果你写:

#include <iostream>

int main() {
  int n;
  std::cin >> n;
  int a[n];
  std::cout << sizeof(a);

  return 0;
}

该程序成功编译(gcc C++17)并打印 n * sizeof(int)。 但为什么会这样呢？

Answer 1

Variable-length arrays不属于标准的一部分。它们可以作为编译器扩展出现，GCC 就是这种情况。编译时您可能会收到以下警告:

warning: ISO C++ forbids variable length array 'a' [-Wvla]

当应用于数组时，sizeof运算符返回整个数组的大小，即基础类型的大小乘以元素数。引用文献指出，我的重点是:

The size of each VLA instance does not change during its lifetime, but on another pass over the same code, it may be allocated with a different size.

标题为 6.19 Arrays of Variable Length 的 GCC 官方文档状态:

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 block scope containing the declaration exits.

话虽如此，更喜欢 std::vector或std::array到原始(C 风格)数组。