CODEWITHSUNDEEP
c++inheritancepolymorphism
Mr Squid
Mr Squid

Reputation: 1276

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

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);
}

Upvotes: 2

Views: 430

Answers (1)

lubgr
lubgr

Reputation: 38267

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.

Upvotes: 5

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