c++ - 初始化变量之前是否调用了父类构造函数？

Question

是在初始化变量之前调用父类的构造函数，还是编译器会先初始化类的变量？

例如:

class parent {
  int a;
public:
  parent() : a(123) {};
};

class child : public parent {
  int b;
public:
            // question: is parent constructor done before init b?
  child() : b(456), parent() {};
}

Answer 1

是的，基类在派生类的成员之前和构造函数体执行之前被初始化。

12.6.2 初始化基和成员 [class.base.init]

In a non-delegating constructor, initialization proceeds in the following order:

— First, and only for the constructor of the most derived class (1.8), virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list.

— Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).

— Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).

— Finally, the compound-statement of the constructor body is executed.