Question

So, I’ve created a basic VC++ program, and created a template class with 1 method (besides the constructor and destructor). I’m getting the following errors:

>main.obj : error LNK2019: unresolved external symbol "public: __thiscall Vector<int>::~Vector<int>(void)" (??1?$Vector@H@@QAE@XZ) referenced in function _main
>main.obj : error LNK2019: unresolved external symbol "public: __thiscall Vector<int>::Vector<int>(int)" (??0?$Vector@H@@QAE@H@Z) referenced in function _main
>c:\users\edy\documents\visual studio 2010\Projects\ex01\Debug\ex01.exe : fatal error LNK1120: 2 unresolved externals

Here is my code:

(CPP Class file)

using namespace std;
#include "Vector.h"
template <class T> Vector<T>::Vector(int n)
{
    this.crt = 0;
    this.dim = n;
    this.elem = new T[100];
}

template <class T> void Vector<T>::add(T e)
{
    this.elem[crt] = e;
    this.crt++;
}

template <class T> Vector<T>::~Vector(void)
{
    this.crt = 0;
}

(H Class file)

#pragma once
template <class T> class Vector
{
    public:
        int dim;
        T* elem;

        Vector(int n);
        void add(T e);
        ~Vector(void);

    private:
        int crt;
};

(Main file)

using namespace std;
#include "Vector.h"

int main(void)
{
    Vector<int> x(5);
    //x.add(1);    <--- if i decomment this line, it throws an additional error
    return 0;
}

Most solutions involved not implemented methods, but I have all my methods implemented. I have no idea what could be wrong. Any help?

Answer 1

Template class implementation need to be visible to all translation units that use them. Move the implementations to the header file.

Before you ask – no, there’s no way to hide them, unless you know in advance which specializations for the class you have. If you want your Vector to be generic, the implementations need to be visible.

If you want to separate them, to usual way to go about it is to have the implementations in a .impl file which you include in the header.