c++ - C++11 是否重新初始化已初始化的成员字段？

Question

C++11 现在支持在声明时设置类成员字段的值，如下所示:

class MyClass
{
private
  int test = 0;
}

如果我也像这样在构造函数中初始化变量:

class MyClass
{
private
  int test = 0;

public:
  MyClass() : test(1)
  {
  }
}

这会导致变量的值设置两次，还是规范规定编译器应该优化它以仅初始化变量一次？如果规范没有规定任何内容，您是否知道著名编译器(例如 MSVC、GCC 等)在这方面的行为？

Answer 1

标准在 §12.6.2/9 中实际上对此有一条规则:

If a given non-static data member has both a brace-or-equal-initializer and a mem-initializer, the initialization specified by the mem-initializer is performed, and the non-static data member’s brace-or-equal-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 brace-or-equal- initializer will not take place. — end example ]

因此在您描述的情况下，如果调用默认构造函数，只会执行那里定义的初始化，并且 test 将为 1。