c++ - 构造函数的调用顺序

Question

#include <iostream>
using namespace std;    
struct A{
  A() {cout << "A" << endl;}
  A(int a) {cout << "A+" << endl;}
};

struct B : virtual A{
  B() : A(1) {cout << "B" << endl;}
};
struct C : virtual A{
  C() : A(1) {cout << "C" << endl;}
};
struct D : virtual A{
  D() : A() {cout << "D" << endl;}
};
struct E : B, virtual C, D{
  E(){cout << "E" << endl;}
};
struct F : D, virtual C{
  F(){cout << "F" << endl;}
};
struct G : E, F{
  G() {cout << "G" << endl;}
};

int main(){
  G g;
  return 0;
}

程序打印:

A
C
B
D
E
D
F
G

我想知道应该使用什么规则来确定调用构造函数的顺序。谢谢。

Answer 1

您应该遵循 C++ 标准中给出的规则:

[C++11: 12.6.2/10]: 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.

[ Note: The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization. —end note ]