Inherited member is not allowed, why? [duplicate]

Question

The basic issue boils down to this:

// header
class Calculator 
{
public:
    virtual int calculate(int a, int b);
};

class Adder : public Calculator 
{

};

// class file
int Adder::calculate(int a, int b) {return a + b;} // gives error

Why am I not allowed to do that?

I would've assumed that since Adder inherits Calculator's members, I can just define their bodies as if they were its own, but apparently that isn't possible...

I've seen other answers in which they simply have to write the member definition again in the child class, but that to me seems like an awful lot of lines of code wasted repeating the same text over and over, so the question is...

Why can't I do it without overriding it in the child class's definition?

Also, please note that this isn't a question as to how to fix an error, it's a question about the mechanics of C++, and why something is or is not permitted to do

Answer 1

You can't declare member functions outside the class definition. Any methods you are going to define for a class (whether overriding or hiding with respect to a base class) must be declared in the class definition.

class Adder : public Calculator { };

Defines a class that derives from Calculator, but provides no additional functionality. Also you probably meant to make the Calculator::calculate method virtual. You are currently (edit: OP edited the question making calculate() virtual) hiding the base class method, rather than overriding it. It makes a difference if you are going to deal with Calculator objects that may in fact be Adder objects at runtime.

Answer 2

You cannot do it because you haven't explicitly said that Adder has a calculate method - and C++ is strict about it. What would happen if you did then write Calculator::calculate in a separate source file?