为什么 const 成员可以被初始化两次？

Question

下面是一个代码片段，可以在vs2015

中编译并且运行没有错误

#include<iostream>
using namespace std;

class A {
    public:
        A(int b) :k(b) {}//second time
    const int k = 666;//first time
};

int main() {
    A a(555);
    cout << a.k << endl;
    return 0;
}

输出为555。但据我所知，const 对象应该只初始化一次，之后该值是不可修改的。

Answer 1

没有初始化两次； default member initializer 只是被忽略了。所以对于A a(555);，a.k被初始化为555.

If a member has a default member initializer and also appears in the member initialization list in a constructor, the default member initializer is ignored.

根据标准，[class.base.init]/10：

If a given non-static data member has both a default member initializer and a mem-initializer, the initialization specified by the mem-initializer is performed, and the non-static data member's default member initializer is ignored. [ Example: Given
struct A {
  int i = /* some integer expression with side effects */ ;
  A(int arg) : i(arg) { }
  // ...
};
the A(int) constructor will simply initialize i to the value of arg, and the side effects in i's default member initializer will not take place. — end example ]

另一方面，给定

class A {
public:
    A() {}            // k will be initialized via default member initializer, i.e. 666
    A(int b) :k(b) {} // k will be initialized via member initializer list, i.e. b

    const int k = 666;
};

那么对于A a;，a.k将被初始化为666。

Answer 2

只初始化一次

const int k = 666;

如果在构造函数中没有提供，将被使用。

为什么 const 成员可以被初始化两次？

Why could const member be initialized twice?

c++

initialization

constants

initializer-list

c++11