Baseclass method is called instead of the one in the subclass

Question

I've created a BaseClass and two subclasses: SubOne and SubTwo. After that I created a collection called MyCollection which stores the instances in a vector.

Both the base class, and the subclasses have the method getString. Base class returns with base, and the subclasses with sub1 and sub2.

I don't get any warning or error during the compilation. But for some reason, if I try to iterate over the vector, the subclasses return "base"

#include <iostream>
#include <vector>

class BaseClass {

    public:

        BaseClass() {}

        std::string getString() {
            return "base";
        }

};

class SubOne : public BaseClass {

    public:

        SubOne() : BaseClass() {}

        std::string getString() {
            return "sub1";
        }

};

class SubTwo : public BaseClass {

    public:

        SubTwo() : BaseClass() {}

        std::string getString() {
            return "sub2";
        }

};

class MyCollection {

    private:

        std::vector<BaseClass> instances;

    public:

        MyCollection() {}


        void add(BaseClass & ins) {
            instances.push_back(ins);
        }

        std::string printString() {

            for(std::vector<BaseClass>::iterator it = instances.begin() ; it != instances.end(); ++it) {
                std::cout << it->getString() << std::endl;
            }

        }

};

int main() {

    MyCollection *coll = new MyCollection();

    SubOne* s1 = new SubOne();
    SubTwo* s2 = new SubTwo();

    coll->add(*s1);
    coll->add(*s2);

    coll->printString();

    return 0;

}

Possible duplicate of [What is object slicing?](http://stackoverflow.com/questions/274626/what-is-object-slicing) — Captain Giraffe, Apr 01 '16 at 20:30
Why wouldn't it? Your function isn't virtual. And more importantly your vector is of `BaseClass`, not `BaseClass &`.so you're slicing. — Rob K, Apr 01 '16 at 20:30
@IterAtor You just don't know what slicing is so you don't know that it is. — Rob K, Apr 01 '16 at 20:35
It's painfully obvious. C++ is nothing like java inspite of superficial similarity. [Get a good C++ book]( http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list?lq=1) and learn C++. — Rob K, Apr 01 '16 at 20:38
This will be helpful: http://stackoverflow.com/questions/22146094/why-should-i-use-a-pointer-rather-than-the-object-itself — Captain Giraffe, Apr 01 '16 at 20:38
I can't really learn from books. I can learn from coding, and my mistakes. Could you please give me some hints, how to create a collection like this? — Iter Ator, Apr 01 '16 at 20:41
Take the time to learn to learn from books. You're going to waste a lot of time otherwise. — Rob K, Apr 01 '16 at 20:46

Riad Baghbanli · Accepted Answer · 2016-04-01T20:56:14.723

1

You forgot to use keyword virtual. Also translate to pointers (credit goes to Captain Giraffe).

See code below:

#include <iostream>
#include <vector>

class BaseClass {

public:

    BaseClass() {}

    virtual std::string getString() { // BINGO _!_!_!_!
        return "base";
    }

};

class SubOne : public BaseClass {

public:

    SubOne() : BaseClass() {}

    std::string getString() {
        return "sub1";
    }

};

class SubTwo : public BaseClass {

public:

    SubTwo() : BaseClass() {}

    std::string getString() {
        return "sub2";
    }

};

class MyCollection {

private:

    std::vector<BaseClass*> instances;

public:

    MyCollection() {}


    void add(BaseClass* ins) {
        instances.push_back(ins);
    }

    std::string printString() {

        for(std::vector<BaseClass*>::iterator it = instances.begin() ; it != instances.end(); ++it) {
            std::cout << (*it)->getString() << std::endl;
        }

    }

};

int main() {

MyCollection *coll = new MyCollection();

SubOne* s1 = new SubOne();
SubTwo* s2 = new SubTwo();

coll->add(s1);
coll->add(s2);

coll->printString();

return 0;

}

edited Apr 01 '16 at 20:56

answered Apr 01 '16 at 20:32

Riad Baghbanli

3,105
1
12
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1

You missed this: `std::vector instances;` He's slicing. – Rob K Apr 01 '16 at 20:33
the BaseClass can also be instantiated, that is why the `getString` is not virtual – Iter Ator Apr 01 '16 at 20:35
@IterAtor That is an invalid argument. There is a big difference between pure virtual and virtual. – Captain Giraffe Apr 01 '16 at 20:36
@rbaghbanli Not fixed. std::vector copies the values into its own container. – Captain Giraffe Apr 01 '16 at 20:39
This is still not a good suggestion for OP. A good answer would replace the Java way with respectable c++ manners. This is, I concede, a tall order. – Captain Giraffe Apr 01 '16 at 20:45
I don't want to ask you to fix my code. I just want to know, how to implement something like this in C++ – Iter Ator Apr 01 '16 at 20:47
In this version you need to `delete s1; delete d2; delete coll;` before `return 0;` in main – M.M Apr 01 '16 at 21:34

Baseclass method is called instead of the one in the subclass

1 Answers1