How call derived version of pure virtual from base?

How would I do this?

Code:

class Base
{
public:
     virtual string getName() const = 0;

     //I want this to call the implemented getName
     string getSomething()
     { return ( "Base." + getName() ); }
};

class Derived : public Base
{
public:
     virtual string getName() const
     { return "Derived"; }
};

In your case, it should simply work as when the derived class derives the functions, you can overload the getName() and it will call its own function (in Derived).
(Which is basically already what you're doing.)

But perhaps there is a more complex usage scenario that you're thinking of?

You need to make the member functions public. At the very least, getSomething() should be public and getName() protected.

Sorry, they are public in my own code, just forgot to write that, but when I try it it throws an error saying I tried to call a pure virtual function!

Code:

//This does NOT work
Derived d();
d.getSomething();

OK, I think I know what the problem is. Can this be "fixed"?

The function getSomething() (which is defined as a base function) is called from the Base constructor. So I think that's why it tries to call the base version:

Code:

class Base
{
public:
     Base()
     { getSomething(); }
}

class Derived : public Base
{
public:
     Derived() : Base() {}
};

I have a bunch of functions that I want called in the Base constructor so every Derived class doesn't have to remember to call them, but I want those functions to call the derived versions. Is this possible?

This works for me:

Code:

#include <iostream>
#include <string>

using namespace std;

class Base
{
public:
     virtual string getName() const = 0;

     //I want this to call the implemented getName
     string getSomething()
     { return ( "Base." + getName() ); }
};

class Derived : public Base
{
public:
     virtual string getName() const
     { return "Derived"; }
};

int main()
{
    Derived x;
    std::cout << x.getSomething() << std::endl;
}

Note that since Base is a polymorphic abstract base class, it should have a virtual destructor.

Also,

Code:

Derived d();

declare a function named d that has no parameters and returns a Derived object.

The function getSomething() (which is defined as a base function) is called from the Base constructor.

That is problematic. Virtual functions should never be called (directly or indirectly) from the constructor.

I have a bunch of functions that I want called in the Base constructor so every Derived class doesn't have to remember to call them, but I want those functions to call the derived versions. Is this possible?

No, not without undefined behaviour, because the Base class constructs first and then the derived.
A workaround might be a common init function in Base that's called in Derived's constructor.

Originally Posted by Elysia

No, not without undefined behaviour, because the Base class constructs first and then the derived.
A workaround might be a common init function in Base that's called in Derived's constructor.

I wouldn't call a virtual function from a constructor even at the same inheritance level. A better solution to this problem would be to use a factory pattern which constructs these objects and then, POST-construction, calls their init() methods.

I was more indicating though that the init & co should not be virtual.
Otherwise a post-init solution may be required.

Thanks! I'll change the way I do it then.

See: Chapter 49. Avoid calling virtual functions in constructors and destructors