调用非静态成员函数而不创建实例

Question

I am a newbie in object oriented approach and C++ programming as well: My question is: How can a class pointer which is not instantiating any object can call the member function of that class. below is the working code which I tried today

#include <iostream>


class Base{
public:
    Base(){
            std::cout << "Base C-tor is called " << std::endl;
    }
    void fun(){
            std::cout << "Base fun() is called " << std::endl;
    }
    void sorrow(){
            std::cout << "Base Sorrow is called " << std::endl;
    }
    ~Base(){
            std::cout << "Base D-tor is called " << std::endl;
    }
};


int main(){

Base *b1;
b1->fun();
b1->sorrow();
}

Below is the output of this piece of code:

Base fun() is called 
Base Sorrow is called 

Answer 1

Despite invoking undefined behavior, your code gives an appearance of working because of the way the compiler invokes non-virtual member functions. In case of your fun() and sorrow() member functions, no access to the instance is performed, so the function completes as if it worked (even though its invocation remains invalid).

If you declare your functions virtual , the likelihood of a crash would go up considerably (although you can't guarantee anything with undefined behavior):

virtual void fun(){
        std::cout << "Base fun() is called " << std::endl;
}
virtual void sorrow(){
        std::cout << "Base Sorrow is called " << std::endl;
}

Calling virtual functions require accessing the instance of the class for the location of the function. Since the pointer is uninitialized, the code is very likely to crash.