template Class .hpp .cpp files

Posted on 2004-08-11
Last Modified: 2012-06-27
I am having trouble separating a templated class into .hpp for the declaration and .cpp for the definition.  Are you not allowed to do this?  My code works when I have it all in the .hpp with nothing but the #include "Fifo.hpp" in the .cpp file.  It will not compile if the definition is in the .cpp file.  Any advice?
Question by:klopex
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LVL 86

Accepted Solution

jkr earned 80 total points
ID: 11776293
>>It will not compile if the definition is in the .cpp file.  Any advice?

Even though the C++ standard allows the separation of template declaration and implementation, most compilers (technically speaking: all compilers) do not support that. As a workaround: Add

#include "fifo.cpp"

to the end of "fifo.hpp", e.g.

#ifndef FIFO_HPP // header guard
#define FIFO_HPP

// declarations go here
// ...
// ...

#include "fifo.cpp"

#endif // header guard

Author Comment

ID: 11776420
I had a #include "Fifo.hpp" at the top of the .cpp file, but it wouldn't work...

How would including the .cpp at the bottom be different?

If I do this, I guess I will need a "header guard" of sorts for the .cpp file as well, yes?
LVL 86

Expert Comment

ID: 11776486
>>How would including the .cpp at the bottom be different?

Because in all files where you include 'fifo.hpp', the .cpp file will be pulled i also, not the other way round.

>>If I do this, I guess I will need a "header guard" of sorts for the .cpp file as well, yes?

No, these only make sense for .hpp/.h files to prevent "symbol xyz multiply defined"  error messages when such a file is included more than once in a project.
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Author Comment

ID: 11776858
I have done what you have suggested, and now I am getting the following errors when I compile:

$ make
g++ -c Fifo.cpp
Fifo.cpp:10: redefinition of `bool Fifo<T>::write(T*, unsigned int)'
Fifo.cpp:10: `bool Fifo<T>::write(T*, unsigned int)' previously declared here
Fifo.cpp:10: no `bool Fifo<T>::write(T*, unsigned int)' member function
   declared in class `Fifo<T>'

Are you sure I should not be using a "header guard"?
LVL 86

Expert Comment

ID: 11776978
It should be 'guarded' by the one in the .hpp file already...

Author Comment

ID: 11777015
It wasn't.  When I compile the Fifo.cpp class, it includes the .hpp which includes the .cpp file.  .cpp is present twice when compiling the .cpp file.  Am I supposed to never do that?

By the way, I put in the guard and it compiles fine both ways.

Thanks for the help.

Assisted Solution

AssafLavie earned 20 total points
ID: 11777020

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