Thread: Template class and unresolved externals..!

Hello all,

I'm experiencing some quite annoying unresolved externals here.

I have the following code:

Code:

// In Foo.h 
#ifndef _FOO_H_
#define _FOO_H_

template <typename T> 
class CFoo
{
private:
	T		m_someT;
public:
	void SetSomeT(T value); // Doesn't work!

	// Works, but not desirable.
	/*void SetSomeT(T value)
	{
		m_someT = value;
	}*/
};

#endif

// In Foo.cpp 
#include "Foo.h"

template <typename T> 
void CFoo<T>::SetSomeT(T value)
{
	m_someT = value;
}

// In Main.cpp 
#include "Foo.h"

int main()
{
	CFoo<int> foo;
	foo.SetSomeT(2);

	return 0;
}

However, I get the following unresolved external error:
Template error LNK2019: unresolved external symbol "public: void __thiscall CFoo<int>::SetSomeT(int)" (?SetSomeT@?$CFoo@H@@QAEXH@Z) referenced in function _main

I know one could "just" move the actual implementation of the template functions into the header (Foo.h) and everything would work out just fine, but WHY?
I mean.. I like keeping my classes and its implementation separated (unless we're talking small accessor functions, etc.)

Can anyone explain why this is messing up the linker?

Oh, and while we're on it.. if I inline my functions in an ordinary class (in the .cpp, too) I get unresolved symbols until I move the actual implementation into the class definition itself. This really doesn't suit me well if I'm doing a .lib and I would like to hide the actual implementation from the user.

Please, I hope you can enlighten me on my C++ path

Thanks in advance!

Or you could include both the .h file and the .cpp file.

Originally posted by CornedBee
NO! NEVER NEVER NEVER include cpp files!

LOL.. thanks for the fast answer(s)!

Seems I'll have to accept the template thing!
What about the inline question? Imagine you're making a .lib and you want to hide the imlementation from the user.. how's that possible if you can't have inline functions in the .cpp file because of the unresolved symbols?

Thanks for the super fast answers!

Just to brush up on this reasonably old thread

Originally Posted by Magos

I usually place the template functions in the header, but if you do't like that put them in an *.ipp file and include that.

.ipp as in "include-pp file" ? How is that any better than including .cpp files?

Also, just a quick rehap. So it is required that all template functions be specified in .h files, like in the class in which they're defined? That is, there is no way to separate the class declaration from the actual definition?

Thanks,

Thanks for the helpful replies, guys. I, too, like to have clean headers and use files with other extensions, such as .cpp, to deal with the actual definitions.

I'll come up with a style of some sort

Originally Posted by kermi3

In fact, so old that I'm going to ask you to make a new thread. Please read the forum rules and don't bump old threads...let them RIP.

Sorry about that. I just figured it would help make the question clearer and that it was just an extension of the original thread.

Originally Posted by CornedBee

What I take issue at is including files with the .cpp ending, because sooner or later, someone or something is going to try and compile them

That's why I use the precompiler directives - to prevent compiling/linking errors. Till now it has worked well.