CODEWITHSUNDEEP
c++templatesg++undefined-reference
user2898624
user2898624

Reputation:

G++ Template instantiation results in "Undefined reference to" error

I've been having some issues lately with my custom written general purpose vector code, which relies on templates for functionality. I am reluctant to include the implementation in the header file (as is common with templates), as this would add significantly to the compilation time. So I have instead manually instantiated the desired class in the .cpp file. However, this still results in an undefined reference error. I have reduced the code to the following snippet, which still generates an error:

matrixd.cpp

#include "matrixd.h"

namespace math
{
    template class _vec2<float>;
    template<class T> _vec2<T>::_vec2() {}
}

matrixd.h

#pragma once
namespace math
{ 
    template <class T>
    class _vec2
    {
    public:
        T x, y;
        _vec2<T>();
        void reset();
    };

    typedef _vec2<float> vec2;
}

test.cpp

#include "matrixd.h"

int main()
{
    math::_vec2<float> v;
}

Error message:

In function main': source.cpp:(.text+0x10): undefined reference to math::_vec2::_vec2()' collect2: error: ld returned 1 exit status

Any help would be appreciated! :)

Upvotes: 1

Views: 1403

Answers (1)

T.C.
T.C.

Reputation: 137310

A explicit instantiation definition (template class _vec2<float>; in your code) instantiates only the member functions that have been defined at the point of explicit instantiation. _vec2<T>::_vec2() was defined after the explicit instantiation definition, and so is not explicitly instantiated.

The fix is to swap the two lines:

namespace math
{
    template<class T> _vec2<T>::_vec2() {}
    template class _vec2<float>;
}

Upvotes: 5

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