C ++继承，调用给定的类函数而不是父类？

Question

Really bad title, couldn't think of how to word it, sorry.

So say I had the following code:

class A {
    virtual int getSize() {
        return 0;
    }
}

class B : public A {
    int getSize() {
        return 32;
    }
}

void doStuff(A a) {
   std::cout << a.getSize() << std::endl;
}

int main() {
   B b;
   doStuff(b);
}

It would print out 0 , however I want it to print out 32 . In other words, I want to pass it the class and it prints out that classes function, so I could create a class C , where the size is 64, and if I pass that C instance to the doStuff function, I want it to print 64.

Is there any way I can do this in C++, would I have to use templates or some fancy C++ feature I don't know about?

Answer 1

A one-byte patch:

void doStuff(A &a) {
  std::cout << a.getSize() << std::endl;
}

Your version takes the argument by value, which means that the function makes a copy of b (a copy which is an A ) and then calls the copy's getSize() . In this version, the function takes the argument by reference, and calls b 's own getSize() , which is B::getSize() .

Answer 2

You should use pointers, or even better: smart pointers! That way, the function of the runtime type gets called. It's a basic example of polymorhpism. If you want to avoid pointers, Beta's slicing approach is equally valid.

#include <iostream>
#include <memory>

class A {
    virtual int getSize() {
        return 0;
    }
}

class B : public A {
    virtual int getSize() {
        return 32;
    }
}

void doStuff(std::shared_ptr<A> a) {
   std::cout << a->getSize() << std::endl;
}

int main() {
   std::shared_ptr<A> b(new B());
   doStuff(b); // Will output '32'.
}  

This should correctly call the function as implemented by B.

Answer 3

Slicing the object is one approach, and in addition I think you're asking for, I think, a pretty straightforward use of polymorphism in C++. http://www.cplusplus.com/doc/tutorial/polymorphism/

That's almost immediately applicable, just call your class A Shape, and B and C could be Square and Triangle. Your DoStuff function could take a pointer to a Shape, then you can pass it a triangle or a square, and when you deference the Shape in the function, it will call the correct function.

So you'd have (also you need to make the members public, I think):

class A {
public:
    virtual int getSize() {
        return 0;
    }
};

class B : public A {

public:
    int getSize() {
        return 32;
    }
};

void doStuff(A* a) {
    std::cout << a->getSize() << std::endl;
}

int main() {
    B b;
    doStuff(&b);
}