How do you properly define a statically inherited method?

Question

I am trying to implement an inheritance chain whereby methods declared on the base type return the derived type by value. For example, I have a base class Base with a method func(). On a derived class Derived, I want func() to return an instance of Derived by value.

To do this, I am trying to use "static polymorphism": the base class takes the derived class via a template parameter, and then makes the method return the template type. Here is my header file:

#pragma once

template<typename Derived>
class Base {
  public:
    Derived func() const;
};

class DerivedA : public Base<DerivedA> {
  public:
    DerivedA() : Base() {}
    DerivedA(const Base<DerivedA>& other) : Base(other) {}
};

So far so good. I define the function in a cpp file:

#include <iostream>

template<typename Derived>
Derived Base<Derived>::func() const {
    std::cout << "Calling func" << std::endl;
    return Derived(*this);
}

And then I call it in a main function:

#include "demo.h"

int main() {
    DerivedA a;
    a.func();
    return 0;
}

Unfortunately, compiling this code results in the following linker error.

main.cpp.o: In function `main': main.cpp:6: undefined reference to `Base<DerivedA>::func() const'
collect2: error: ld returned 1 exit status

Here is my CMakeLists.txt:

cmake_minimum_required(VERSION 3.7)
project(demo)

set(CMAKE_CXX_STANDARD 11)

set(SOURCE_FILES main.cpp demo.cpp demo.h)
add_executable(demo ${SOURCE_FILES})

Oddly, if I move the definition into the header file, the code compiles without the linker error.

How do I properly define the function (ideally in a cpp file) and make the linker error go away?

How do you properly define a statically inherited method?

0 Answers0