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?

Answer 1

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.