Question

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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++

Answer 1

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: 
....