//Base.h
#pragma once
class Base
{
public:
    void foo();
    virtual void bar();
};

//Base.cpp
#include "Base.h"
void Base::foo()
{
}


//Derived.h
#pragma once
#include "Base.h"
class Derived : public Base
{
public:
    void bar() override;
}

//Derived.cpp
#include "Derived.h"

void Derived::bar()
{
}

//main.cpp
#include "Derived.h"

int main(/**/)
{
    Derived d;
    Base* ptr = &d;

    d->foo();
    d->bar();

    return 0;
}