C++ class template undefined reference to function [duplicate]

Answer 1

I prefer to have all of my functions in the .cpp file, regardless of whether they are template functions or regular functions. And there is a way to do that with some basic #ifndef magic. Here's what you can do:

main.cpp

#include "myclass.hpp"

int main()
{
  // ...
}

myclass.hpp

#ifndef MYCLASS
#define MYCLASS

template
class MyClass
{
  T val;
public:
  MyClass(T val_);
}

#define MYCLASS_FUNCTIONS
#include "myclass.cpp"

#endif

myclass.cpp

#ifndef MYCLASS_FUNCTIONS
#include "myclass.hpp"

// regular functions:
// ...

#else

// template functions:
template
MyClass::MyClass(T val_)
    :val(val_)
{}

// ...
#endif

Here's how the precompiler sees it. We have two .cpp files.

1. When we compile main.cpp we:
   1. include myclass.hpp
   2. check that MYCLASS is undefined, and it is
   3. define it
   4. give compiler the definitions of the generated class (from template class)
   5. include myclass.cpp
   6. define MYCLASS_FUNCTIONS
   7. check if MYCLASS_FUNCTIONS is defined, it is
   8. give compiler the definitions of the generated functions (from template functions)
2. When we compile myclass.cpp
   1. check if MYCLASS_FUNCTIONS is defined, it isn't
   2. include myclass.hpp
   3. check that MYCLASS is undefined, and it is
   4. define it
   5. give compiler the definitions of the class
   6. include myclass.cpp
   7. include myclass.hpp again
   8. this time MYCLASS is defined so do nothing inside, return to myclass.cpp
   9. check if MYCLASS_FUNCTIONS is defined, it is
   10. give compiler the definition of the generated functions (from template functions)
   11. exit include twice
   12. pass to the compiler all the regular functions