c++ - 如何在 C++ 类的初始化列表中初始化成员结构？

Question

我在 c++ 中有以下类定义:

struct Foo {
  int x;
  char array[24];
  short* y;
};

class Bar {
  Bar();

  int x;
  Foo foo;
};

并希望在 Bar 类的初始化程序中将“foo”结构(及其所有成员)初始化为零。可以这样做吗:

Bar::Bar()
  : foo(),
    x(8) {
}

... ?

或者 foo(x) 在初始化列表中究竟做了什么？

或者编译器是否自动将结构初始化为零？

Answer 1

首先，您应该(必须!)阅读 c++ faq关于 POD 和聚合。在您的情况下， Foo 确实是一个 POD 类，而 foo() 是一个 值初始化 :

To value-initialize an object of type T means:

• if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor
  for T is called (and the initialization is ill-formed if T has no accessible default constructor);
• if T is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;
• if T is an array type, then each element is value-initialized;
• otherwise, the object is zero-initialized

所以是的，foo 将被零初始化。请注意，如果您从 Bar 构造函数中删除了此初始化，则 foo 只会是 default-initialized :

If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor. Otherwise, if no initializer is specified for a nonstatic object, the object and its subobjects, if any, have an indeterminate initial value;