c++ - 从成员构造函数调用虚函数

Question

我对以下行为感到好奇:

#include <iostream>
#include <string>
struct A;
struct B {
    std::string b;
    B(A& a);
};
struct A {
    B member;
    virtual std::string f() { return "Hello, World!"; }
    A() : member(*this) {}
};
B::B(A& a) : b(a.f()) {}
int main() {
    std::cout << A().member.b;
}

这是打印预期结果的必需吗？还是未定义的行为？

Answer 1

这是合法的。 §12.7 [class.cdtor]/p4:

Member functions, including virtual functions (10.3), can be called during construction or destruction (12.6.2). When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class’s non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor’s or destructor’s class and not one overriding it in a more-derived class. If the virtual function call uses an explicit class member access (5.2.5) and the object expression refers to the complete object of x or one of that object’s base class subobjects but not x or one of its base class subobjects, the behavior is undefined.

UB 案例不适用于此。