How does resolution of class member identifiers works in c++

Question

Say you have a class Ball and in the Ball.h you have

class Ball {
public:
    int foo();
};

with its Ball.cc file having the function

int Ball::foo(){
    return 5;
}

and then you have a class BigBall with

#include <Ball.h>

class BigBall : public Ball {
public:

};

and then in your main you make a BigBall

auto bigRed = make_shared<BigBall>():

how would you go about using foo(); on the bigRed object?

somthing like

Ball::bigRed->foo();

but I get told that ball cant be resolved....

You are using `auto`, `make_shared` but you have no idea what inheritance is? I would advice you to start with a beginner book to learn the basics. — Klaus, Dec 17 '16 at 17:40

score 1 · Accepted Answer · answered Dec 17 '16 at 17:40

1

Almost. You need to do the resolution on the function identifier, not the object:

bigRed->Ball::foo();

But unless BigBall overloads and hides the original Ball::foo, you don't really need to resolve anything:

bigRed->foo();

answered Dec 17 '16 at 17:40

StoryTeller - Unslander Monica

165,132
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hmm im still getting an error tho, that its an invalid use of Ball::Ball and that method foo could not be resolved – sam Dec 17 '16 at 17:46
and your second way states an undefined reference with Ball::foo(); – sam Dec 17 '16 at 17:48
@sam See this http://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix – StoryTeller - Unslander Monica Dec 17 '16 at 17:48
hmmm maybe that's the case i'm pretty new to this and just using the eclipse builder. but without trying to use inheritance here my program runs fine with about 20 classes in it – sam Dec 17 '16 at 17:50
@sam If a class `BigBall` inherits from `Ball`, then `Ball` must be linked into `BigBall`. That's how the c++ object model works. – StoryTeller - Unslander Monica Dec 17 '16 at 17:52
ahhhhhhhhh yes your right it works perfectly now thank you so much :) – sam Dec 17 '16 at 17:56

score 1 · Answer 2 · answered Dec 17 '16 at 17:41

1

With std::shared_ptr you can use the -> operator like this:

auto foo = bigRed->foo();

Enjoy!

answered Dec 17 '16 at 17:41

Stefano Azzalini

1,027
12
21

score 1 · Answer 3 · answered Dec 17 '16 at 17:52

All you need is this:

BigBall ball;
ball.foo();

That creates an object named ball of type BigBall and calls foo(). You don't have to do anything special to call a function that's defined in a base class. Just call it.

That auto and make_shared stuff is just a distraction for now.

How does resolution of class member identifiers works in c++

3 Answers3