instantiating a class template and call its methods

Question

Possible Duplicate:
Why can templates only be implemented in the header file?

When I include MyClass.h and do:

MyClass<int, int> ccc = MyClass<int, int>();
ccc.myMethod1(3, 4);

I get a lot of errors telling undefined reference to constructor and methods ... However when I include MyClass.cpp (which is not a proper why to code) there is no error ! How to fix that ?

I'm compiling under Code::Blocks using g++

See [Why can templates only be implemented in the header file?](http://stackoverflow.com/q/495021/20984). — Luc Touraille, Jan 25 '12 at 10:40
@chac is there any difference between **MyClass ccc;** and **MyClass ccc = MyClass();** ? It's like **vector v;** and **vector v = vector();** it's the same thing I think. — shn, Jan 25 '12 at 11:28

score 2 · Accepted Answer · answered Jan 25 '12 at 10:38

The reason is that template classes are not compiled, only instantiated templates will be compiled.

Rule of thumb: don't place template implementations in a cpp file but either directly in the header or another file included by the header (if you want to part implementation from the interface).

E.g:

myclass.h

template<typename A>
class MyClass
{
    ...
};

#include "myclass.inc"

myclass.inc

//implementation goes here: 
....

instantiating a class template and call its methods

1 Answers1