Lineker has template class problem (LNK2001)

[IonelBurtan]

I haven't used template classes for quite a while and now, when I am tring to use it I get a linker problem.

I made a template class


template <class T> class CSearchAlgorithms
{
public:
CSearchAlgorithms(T* pArray, int nCount, T nLookedFor);
virtual ~CSearchAlgorithms();
void LinearSearch();
void BinarySearch();

private:
T* m_pArray;
int m_nCount;
T m_nLookedFor;
};

and place it in SearchAlgorithms.h
the body of the template class is placed in SearchAlgorithms.cpp with funtion declaration like this
like this

template <class T>
void CSearchAlgorithms<T>::LinearSearch()
{
...
}


Then I try to use the template class from the main program. I include SearchAlgorithms.h, and I instantiate with

...
CSearchAlgorithms<int> sa(nArray, nArraySize, nLookedFor);
sa.LinearSearch();
sa.BinarySearch();
...

All compiles well but the linker gives me 4 errors (i have 4 methods in the template class) like this:

SearchProj.obj : error LNK2001: unresolved external symbol &quot;public: virtual __thiscall CSearchAlgorithms<int>::~CSearchAlgorithms<int>(void)&quot; (??1?$CSearchAlgorithms@H@@UAE@XZ)
SearchProj.obj : error LNK2001: unresolved external symbol &quot;public: void __thiscall CSearchAlgorithms<int>::BinarySearch(void)&quot; (?BinarySearch@?$CSearchAlgorithms@H@@QAEXXZ)
SearchProj.obj : error LNK2001: unresolved external symbol &quot;public: void __thiscall CSearchAlgorithms<int>::LinearSearch(void)&quot; (?LinearSearch@?$CSearchAlgorithms@H@@QAEXXZ)
SearchProj.obj : error LNK2001: unresolved external symbol &quot;public: __thiscall CSearchAlgorithms<int>::CSearchAlgorithms<int>(int *,int,int)&quot; (??0?$CSearchAlgorithms@H@@QAE@PAHHH@Z)
Debug/SearchProj.exe : fatal error LNK1120: 4 unresolved externals

I am sure I am missig something but i do not remember what
Tnx,

s-)

Blessed is he who in the name of justice and goodwill, sheperds the weak through the valley of darkness...

http://www20.brinkster.com/ionelb

 

[B00gyeMan]

Common problems:

1) A project that uses function inlining yet defines the functions in a .cpp file rather than in the header file can cause LNK2001.

2) Attempting to reference functions or data that don't have external linkage can cause LNK2001. In C++, inline functions and const data have internal linkage unless explicitly specified as extern.

3) A missing function body or variable can cause LNK2001.

4) Calling a function with parameter types that do not match those in the function declaration can cause LNK2001. Name decoration incorporates the parameters of a function into the final decorated function name.

5) When using C++, including a function prototype in a class definition and failing to include the implementation of the function for that class can cause LNK2001.

All 5 above are taken from MSDN. I've had this problem too, but solved it by implementing the destructor (that's it, i forgot :-D )

 

[IonelBurtan]

Tnx pal, but I solved it. It is a non-compliance of the Visual C++ 6 with the C++ standard for templates.

In MSDN they describe it in the article
PRB: LNK2001 on Template Member Functions.

Both of the work-arounds proposed work, but i chose the first, that is to put both the declaration and implementation in the same file and name it .h or no extension

Tnx,


s-)

Blessed is he who in the name of justice and goodwill, sheperds the weak through the valley of darkness...

http://www20.brinkster.com/ionelb

 

[OlafBogus]

Don't include the *.h include the *.cpp that should sort it.

 

[mikefarr]

I had this same problem. I tried putting all the inplementation of the class in the header file. It worked partly. But MFC needs that MESSAGE_MAP declaration be in the cpp file and so the compiler is now complaining. Has anyone found a workaround for this?

Thanks a lot
Mike

 

[JonnoA]

If you want to put the implementation in the .cpp file and you know what the template parameters will be, put the following at the top of your .cpp file:

#pragma warning(disable: 4660)

template class YourTemplate<double>;
template class YourTemplate<complex<double> >;

etc

JonnoA

 

[Minsk]

All that you are talking about is clear, but I have more interesting case.

I am trying compile my c++ application under .NET. And get a lot of errors like following:

McAttClip.obj : error LNK2001: unresolved external symbol &quot;__declspec(dllimport) public: __thiscall std::basic_string<unsigned short,struct std::char_traits<unsigned short>,class std::allocator<unsigned short> >::~basic_string<unsigned short,struct std::char_traits<unsigned short>,class std::allocator<unsigned short> >(void)&quot; (__imp_??1?$basic_string@GU?$char_traits@G@std@@V?$allocator@G@2@@std@@QAE@XZ)

I wonder that it wants?
basic_string???!!!
I use wstring and have no problem until this moment.