C++ Inheritance: Pass sub-class to a function expecting base-class and get sub-class behaviour

Question

In the following toy example, I have a base class and a class which inherits from it. The child class overrides the method doSomething(). However when I pass an instance of the child class to a function expecting the base class, I get the behavior of the base class, i.e the output is I'm the base class! rather than I'm the child class!. How can I prevent this from happening and why does it happen (I guess there is some sort of implicit casting going on?).

Many thanks!

#include <iostream>

using namespace std;

class BaseClass {
public:
    void doSomething() {
        cout << "I'm the base class!";
    }
};

class SubClass : public BaseClass {
public:
    void doSomething() {
        cout << "I'm a child class!";
    }
};

void thisFunction(BaseClass & instance) {
    instance.doSomething();
}

int main(int, char*[]) {
    SubClass sc;
    thisFunction(sc);
}

I recommend you [get a good C++ book or two to read](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list/388282#388282), and read about *polymorphism* and **`virtual`** functions. — Some programmer dude, Oct 23 '18 at 07:16
There's no such thing as "implicit casting". There's explicit and implicit *conversion*, and casting is explicit conversion. — molbdnilo, Oct 23 '18 at 07:23

lubgr · Accepted Answer · 2018-10-23T07:24:53.980

Your BaseClass::doSomething() is not virtual. This way, it can't be overridden, but only shadowed. Try changing its signature to

class BaseClass {
    virtual void doSomething() { /* ... */ }
};

No changes are required for the SubClass, it is, however, good practice to make use of the override keyword (your example code is a perfect illustration for this - marking the method with override makes the compiler complain if the exact same function signature isn't virtual in any base class) :

class SubClass : public BaseClass {
    void doSomething() override { /* ... */ }
};

In addition to that, please make the BaseClass destructor virtual, too (or protected), or you risk undefined behavior when deleting a SubClass instance through a BaseClass pointer.

Thank you, this works. More information (for those that want it) can be found here: https://www.geeksforgeeks.org/virtual-function-cpp/ — Mr Squid, Oct 23 '18 at 07:23

C++ Inheritance: Pass sub-class to a function expecting base-class and get sub-class behaviour

1 Answers1