Implementation of templated virtual function in c++ outside the header

Question

I have a simple cmake project where I'm implementing a virtual function f() outside the header file base.h, in base.cpp. Here's base.h

#include <iostream>

template<typename T>
class Base {
public:
    virtual void f(const T& t) const = 0;
};

template<typename T>
class Derived : public Base<T> {
public:
    void f(const T& t) const final;
};

and base.cpp

#include "base.h"

template <typename T>
void Derived<T>::f(const T& t) const {
    std::cout << t << std::endl;
}

and run via main.cpp

#include <iostream>

#include "base.h"

int main() {
    Derived<double> e;
    e.f(2);
}

My CMakeLists.txt reads:

cmake_minimum_required(VERSION 3.21)

project(MyProject)

set(CMAKE_CXX_STANDARD 17)

add_executable(MyProject main.cpp base.cpp)

However I get the following error message

Undefined symbols for architecture arm64:
  "Derived<double>::f(double const&) const", referenced from:
      _main in main.cpp.o
      vtable for Derived<double> in main.cpp.o
ld: symbol(s) not found for architecture arm64

The code compiles successfully if I instead implement the virtual function in base.hpp in-place, instead of externally in base.cpp. It also compiles if I remove the template <typename T> specification for the abstract Base class. What am I missing here, and how do I write the virtual function's implementation in a separate file?

Implementation of templated virtual function in c++ outside the header

0 Answers0