Trouble with inheritance in C++

Question

I'm having trouble getting the std list class to call the proper function on a class. It keeps calling the base class' function only. Here's what's happening:

I have a state class with the following update function which is declared virtual in the header:

State.h update function:

virtual void update(float elapsed);

State.cpp update function:

void State::update(float elapsed){

};

and a class that inherits from it called TestState:

class TestState::public State{
        virtual void update(float elapsed){
             if (GLOBALENGINE->getInput() -> getKeyPress(DIK_Q)
                PostQuitMessage(0);
        };

};

I call the addState function to add a new state to the linked list and make it the current state as follows:

GLOBALENGINE->addState(new TestState,true);

The addState function looks like this:

void GameEngine::addState(State *state, bool change){
    states->push_back(*state); //add the test state into the states list

    // some irrelevant code

    currentState = &(states->back());//currentState is a pointer to a State
    // So we make it point to the newly added state.


}

Then the run() function is called every frame and inside of it is the following code:

currentState->update(elapsed)// This should call the update function in TestState!

I thought that this line should call the update function in TestState but instead it calls only the function in the State class (ie: it does nothing).

Can someone explain how I can get the functionality I'm looking for?

Thanks

How is the `states` list declared? – ibid Dec 03 '11 at 07:46 — ibid, Dec 03 '11 at 07:46

score 5 · Accepted Answer · edited May 23 '17 at 12:27

You should store in the list a pointer (or a smart pointer) to your object, not an object directly, or you lose polymorphism. In

states->push_back(*state);

you are instructing push_back to construct a copy of state and add the copy in states. The copy is of type State, not TestState, and thus does not behave the same way.

This behaviour is called object slicing; see for example this SO question.

Trouble with inheritance in C++

1 Answers1