C++'s pure virtual function implementation and header files

You forgot to declare Derived::method().

You tried to define it at least, but wrote Derived::Interface::method() rather than Derived::method(), but you did not even attempt to declare it. Therefore it doesn't exist.

Therefore, Derived has no method(), therefore the pure virtual function method() from Interface was not overridden... and therefore, Derived is also pure virtual and cannot be instantiated.

Also, public void method()=0; is not valid C++; it looks more like Java. Pure virtual member functions have to actually be virtual, but you did not write virtual. And access specifiers are followed by a colon:

public:
    virtual void method() = 0;

You have to declare your method in the subclass.

// interface.hpp
class Interface {
public:
    virtual void method()=0;
}

// derived.hpp
class Derived : public Interface {
public:
    void method();
}

// derived.cpp
void
Derived::method()
{
    // do something
}