Overriding parent class's function

Question

class classa {
public:
    virtual void foo();
};

class classb : public classa {
public:
     virtual void foo() override;
};


void classa::foo()
{
    std::cout << "foo from a" << std::endl;
}

void classb::foo()
{
    std::cout << "foo from b" << std::endl;
}

int main()
{
    std::vector<classa> stuff; 

    classa a;
    classb b;

    stuff.push_back(a);
    stuff.push_back(b);

    stuff[0].foo();
    stuff[1].foo();


    return 0;
}

I expected the above code to return

foo from a 
foo from b

but it returns both as foo from a.

I think this is because the vector stores classa but I am not sure. How can I get classb:foo() to be called by b?

"std::cout stuff;" seems wrong, are you sure that's really your code? — Pete Fordham, Sep 20 '12 at 19:12
http://stackoverflow.com/questions/274626/what-is-the-slicing-problem-in-c — chris, Sep 20 '12 at 19:13
@PeteFordham stackoverflow changed it for some reason. It is meant to read (this might change too...): std::vector stuff; — , Sep 20 '12 at 19:15
You can't use `<` and `>` in a `
` because it is interpreted as HTML. Instead, use four space indenting. — Raymond Chen, Sep 20 '12 at 19:19

score 7 · Accepted Answer · answered Sep 20 '12 at 19:11

7

This happens because of object slicing, you'll need to keep a vector of pointers (preferably smart pointers).

I'm assuming stuff is defined as std::vector<classa> stuff;. When you do

stuff.push_back(b);

the object pushed into the vector is a slice of b - particulary the classa part. All other type info is lost. For this to work as expected, you'd need:

std::vector<classa*> stuff;

or similar. The way your code is now, you can't get it to work because stuff[1] is no longer a classb, but a classa.

answered Sep 20 '12 at 19:11

Luchian Grigore

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I have to use `stuff.push_back(&a);`. That is right, right? – Sep 20 '12 at 20:34
@andrew either that, or create the object with `new` (and `delete` it afterwards). – Luchian Grigore Sep 20 '12 at 20:58

Overriding parent class's function

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