How to create custom C++ template that receives any type such as STL vector?

Question

I'm trying to create data structure for sorting data that will firstly receive any kind of object, i.e. int, string or MyCustomObject and then perform other tasks.

// heap.h
template <typename T>
class Heap{
public:
    Heap();
    ~Heap();    
}


//heap.cpp
template <typename T>
Heap<T>::Heap(){}

template <typename T>
Heap<T>::~Heap(){}

...

It turns out that from the very beginning when I try to run Heap<int> test it shows that classic message undefined reference to Heap<int>::Heap(). I found out that one workaround is to add template <typename int> at the end of the code.

However, if I try to do it with std::vector I'm pretty sure I can run std::vector<MyCustomObject> test which uses templates even without adding that line template<typename MyCustomObject> beforehand.

I want to know:

1. How would I perform this same behavior in my data structure of receiving "unknown" types?
2. Why std::vector implementation works whereas mine don't? What's missing?

Answer 1

To remove the undefined reference to Heap<int>::Heap() error you have to put the definition into the header file, too. Yes, that may seem strange, but that is the usual approach with templates:

heap.hpp:

template <typename T>
class Heap{
public:
    Heap();
    ~Heap();
};

template <typename T>
Heap<T>::Heap(){}

template <typename T>
Heap<T>::~Heap(){}

main.cpp:

#include "heap.hpp"

int main()
{
  Heap<int> test_with_int;
  Heap<double> test_with_double;
  return 0;
}