How do I define a template class and divide it into multiple files?

Question

I have written a simple template class for test purpose. It compiles without any errors, but when I try to use it in main(), it give some linker errors.


main.cpp

#include <iostream>
#include "MyNumber.h"

int wmain(int argc, wchar_t * argv[])
{
    MyNumber<float> num;
    num.SetValue(3.14);
    std::cout << "My number is " << num.GetValue() << "." << std::endl;

    system("pause");
    return 0;
}

MyNumber.h

#pragma once

template <class T>
class MyNumber
{
    public:
        MyNumber();
        ~MyNumber();
        void SetValue(T val);
        T GetValue();

    private:
        T m_Number;
};

MyNumber.cpp

#include "MyNumber.h"

template <class T>
MyNumber<T>::MyNumber()
{
    m_Number = static_cast<T>(0);
}

template <class T>
MyNumber<T>::~MyNumber()
{
}

template <class T>
void MyNumber<T>::SetValue(T val)
{
    m_Number = val;
}

template <class T>
T MyNumber<T>::GetValue()
{
    return m_Number;
}

When I build this code, I get the following linker errors:

Error 7 Console Demo C:\\Development\\IDE\\Visual Studio 2010\\SAVE\\Grand Solution\\X64\\Debug\\Console Demo.exe 1 error LNK1120: 4 unresolved externals

Error 3 Console Demo C:\\Development\\IDE\\Visual Studio 2010\\SAVE\\Grand Solution\\Console Demo\\main.obj error LNK2019: unresolved external symbol "public: __cdecl MyNumber::~MyNumber(void)" (??1?$MyNumber@M@@QEAA@XZ) referenced in function wmain

Error 6 Console Demo C:\\Development\\IDE\\Visual Studio 2010\\SAVE\\Grand Solution\\Console Demo\\main.obj error LNK2019: unresolved external symbol "public: __cdecl MyNumber::MyNumber(void)" (??0?$MyNumber@M@@QEAA@XZ) referenced in function wmain

Error 4 Console Demo C:\\Development\\IDE\\Visual Studio 2010\\SAVE\\Grand Solution\\Console Demo\\main.obj error LNK2019: unresolved external symbol "public: float __cdecl MyNumber::GetValue(void)" (?GetValue@?$MyNumber@M@@QEAAMXZ) referenced in function wmain

Error 5 Console Demo C:\\Development\\IDE\\Visual Studio 2010\\SAVE\\Grand Solution\\Console Demo\\main.obj error LNK2019: unresolved external symbol "public: void __cdecl MyNumber::SetValue(float)" (?SetValue@?$MyNumber@M@@QEAAXM@Z) referenced in function wmain

But, if I leave main() empty, I don't get any linker errors.

What is wrong with my template class?
What am I doing wrong?

Answer 1

You have to instantiate your template explicitly for each template parameter you use.

Ie, add the following line at the end of the MyNumber.cpp file:

template class MyNumber<float>;

This way, the linker will be able to find all template instantiations it needs.

See also Moving Templates Out of Header Files .

Answer 2

You can not implement the template in a cpp file. You need to define the class methods in the header file itself. See this Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file? for more information.

Answer 3

Well, you're actually supposed to be able to use the export keyword, but almost no compilers implement it. You can work around this by extracting non-generic code into separate functions, and define these in separate files.