问C++11中类数据成员的默认初始化

EN

#include <iostream>
#include <vector>

class A {
public:
  int i;
  A() {}
};

A a1;
A aa1[3];
std::vector<A> av1(3);

int main()
{
  A a2;
  A aa2[3];
  std::vector<A> av2(3);
  std::cout << a1.i << " " << a2.i << std::endl;          // 0         undefined
  std::cout << aa1[0].i << " " << aa2[0].i << std::endl;  // 0         undefined
  std::cout << av1[0].i << " " << av2[0].i << std::endl;  // undefined undefined
}

- `a1` and `a2` are _default initialized_.
- every element of `aa1` and `aa2` are _default initialized_.
- every element of `av1` and `av2` are _value initialized_.

- check if the variable is _built-in type_ or _class type_.
- for _built-in type_, if the variable is outside any function body, then it is initialized to 0, else the value is undefined.
- for _class type_, if the class has default ctor, then that is called, else it is a compile error.

- check if the variable is _built-in type_ or _class type_.
- for _built-in type_, it is initialized to 0.
- for _class type_, it is _default initialized_. (Which I think means if the class has default ctor, then that is called, else it is a compile error.)