C++ - Static templated method inside a class

phil o

So, I finally decided to dive into C++ world, trying to adapt some C# code I've been working on lately. So here's what I have. bin.h file:

#pragma once

namespace Maths {

class Bin
{
public:
template static const T GetMask(int);
};

}

bin.cpp file:

#include "bin.h"

namespace Maths {

template const T Bin::GetMask(int count) {
return (T)0; // Fake implementation
}

}

This compiles just fine. However, in the same console project, when I write:

#include "bin.h"

using namespace Maths;

int main() {
unsigned char value = Bin::GetMask<unsigned char>(0);
}

I get a nasty LNK2019 linkage error on compilation:

Error LNK2019 unresolved external symbol "public: static unsigned char const __cdecl Maths::Bin::GetMask(int)" (??$GetMask@E@Bin@Maths@@SA?BEH@Z) referenced in function "int __cdecl main(void)" (?main@@YAXXZ)

In the Bin class, I have a couple of other (non-generic) methods, which I can use just fine. I suspect a problem with static templated methods specifically, but I can't understand the core problem. Maybe I just try to do something which is not possible (using a static generic method), but I cannot figure out why it would not be possible. Anyone could give me a hint on this?

"I'm neither for nor against, on the contrary." John Middle

Lost User

The Maths namespace does not contain a definition/declaration of Bin::GetMask(int); only one of Bin::GetMask(int);. Just remove the <unsigned char> piece and it should be fine.

phil o

Thank you Richard for answering. However, if I try to remove the <unsigned char> piece (namely: unsigned char value = Bin::GetMask(0);), the editor itself complains about the fact that no instance of function template "Maths::Bin::GetMask" matches the argument list. Moreover, the Bin::GetMask(int) is actually declared in Maths namespace. Problem seems to be with the .cpp part: the editor/IDE can see the definition of the generic method (since the editor does not complain when I try to use it, and the object explorer clearly displays the method with correct signature), but, on compilation, the linker cannot make a link to the actual implementation. How can I correctly declare a static, generic method inside a non-generic class, then? Thanks for your time :)

"I'm neither for nor against, on the contrary." John Middle

phil o

Anyway, I found the issue. I cannot implement a template method in the .cpp file, like I did with other non generic methods. I moved the implementation of the generic method in the .h header file, and now everything is ok. I guess I have to dig deeper in the translation unit concept :)

"I'm neither for nor against, on the contrary." John Middle

CPallini

Yes, that's it. Template method definitions should be in header files.

Lost User

phil.o wrote:

the Bin::GetMask<T>(int) is actually declared in Maths namespace.

I don't see it anywhere, unless that is somehow implied by template static const T GetMask(int);. Hmm, I though I understood templates.

Lost User

phil.o wrote:

the translation unit concept

The only reason for creating header files is when you have a set of common definitions that need to be shared by multiple translation units. If you only have a single .cpp file then all the definitions can safely be put in there. The #include statement is recognised by the preprocessor which reads all the include files and creates a new text file containing all the included code followed by the user written C/C++ code, and passes that new composite to the actual compiler.

phil o

I have learnt it the hard way :) This explains why everything was fine as far as the editor/IDE were concerned, and why the issue only occurred on the linking phase. That's quite confusing for a beginner.

"I'm neither for nor against, on the contrary." John Middle

Lost User

phil.o wrote:

quite confusing for a beginner.

and sometimes for the rest of us.