Question

I have a problem with generics in c++.I have two Matrix.h and Matrix.cpp files.Here is files:

#pragma once
template<class T>
class Matrix
{
    public:
        static T** addSub(int size,T** firstMatrix,T** secondMatrix,int operation);
}

and Matrix.cpp

#include "Martix.h"
template<class T>
static T** Matrix<T>::addSub( int n,T **firstMatrix,T **secondMatrix,int operation)
{
    //variable for saving result operation
    T **result = new T*[n];

    //create result matrix
    for( int i=0;i<n;i++)
        result[i] = new T[n];

    //calculate result
    for( int i=0;i<n;i++)
        for(int j=0;j<n;j++)
            result[i][j] = 
            (operation == 1) ? firstMatrix[i][j] + secondMatrix[i][j]:
                                firstMatrix[i][j] - secondMatrix[i][j];

    return result;
}

when I run these I get the below error:

Error   1   error LNK2019: unresolved external symbol "public: static int * * __cdecl Matrix<int>::addSub(int,int * *,int * *,int)" (?addSub@?$Matrix@H@@SAPAPAHHPAPAH0H@Z) referenced in function "public: static int * * __cdecl Matrix<int>::strassenMultiply(int,int * *,int * *)" (?strassenMultiply@?$Matrix@H@@SAPAPAHHPAPAH0@Z) C:\Users\ba.mehrabi\Desktop\Matrix\matrixMultiplication\main.obj    matrixMultiplication

what is the problem?

Answer 1

Unfortunately, you can’t have the declaration of a template class in the *.cpp files. The full definition has to stay in the header file. This is a rule of many C++ compilers.

See this: http://www.parashift.com/c++-faq-lite/templates.html#faq-35.12

1. A template is not a class or a function. A template is a “pattern”
   that the compiler uses to generate a family of classes or functions.
2. In order for the compiler to generate the code, it must see both
   the template definition (not just declaration) and the specific
   types/whatever used to “fill in” the template. For example, if you’re
   trying to use a Foo, the compiler must see both the Foo template
   and the fact that you’re trying to make a specific Foo.
3. Your compiler probably doesn’t remember the details of one .cpp
   file while it is compiling another .cpp file. It could, but most do
   not and if you are reading this FAQ, it almost definitely does not.
   BTW this is called the “separate compilation model.”

The link has a workaround, but keep in mind that a template is a glorified macro so the header file is probably the best place for it.

The poster of this SO post shows a demonstration of the workaround.