In C++, how can I test if an object is an instance of a Child class in the Parent class' constructor?

Question

I'm trying to assert that a pointer passed into a Parent class' constructor is only NULL if the object is of a specific Child class type using dynamic_cast :

#include <iostream>
class Parent {
public:
    Parent(void *ptr);
    virtual ~Parent(); // to make Parent polymorphic
};

class Child1 : public Parent {
public:
    Child1() : Parent(0) { std::cout << "Child1 ctor\n";};
};
class Child2 : public Parent {
public:
    Child2() : Parent(0) { std::cout << "Child2 ctor\n";};
};

Parent::Parent(void *ptr) {
    if (0 == ptr && 0 == dynamic_cast<Child1*>(this)) {
         std::cerr<<"ERROR\n";
    }
}
Parent::~Parent() {};

int main(void) {
    Child1 *c1 = new Child1();
    Child2 *c2 = new Child2();
}

This prints:

ERROR
Child1 ctor
ERROR
Child2 ctor

Whereas, I expect to see ERROR during the Child2 construction only .

Why is dynamic_cast returning non-NULL when I'm in Parent constructor for Child1 called from Child1 's constructor initialization list? Also, is there a different way to accomplish this test?

Answer 1

When you are in Parent constructor -which is a base class- the Child has not been constructed yet. As a result, within the constructor of Parent the dynamic type of this will always be Parent.

Answer 2

I'd guess the issue is that dynamic_cast works off the vtable, which isn't set up until after the constructor finishes. Thus, you can't call dynamic_cast in the constructor.

I can't think of any immediate ways to detect this in the constructor without using templates and making everything static. Why do you want this behaviour? It seems quite dubious - Parent really should have no knowledge of the derivatives Child1 and Child2.

If you want to protect against a null pointer in when Child2 calls the parent constructor, why not just protect against it in Child2's constructor and throw an exception if it is null?

Answer 3

进行这种瘦削的一种方法是在基类中创建预期的构造函数，并传递指示如何初始化它的参数-这样子类将决定他们想要的方式，而基类将根据需要进行布置。