Code duplication between typedefs and explicit instantiations

Question

tree.h

template
class binary_operation : public node
{
// ... unimportant details ...

    unsigned evaluate() const;
         


        

Answer 1

Use a macro. You could write a header like

I_HATE_MACROS(binary_operation<std::plus<unsigned>, '+'>, addition)
I_HATE_MACROS(binary_operation<std::multiplies<unsigned>, '*'>, multiplication)

Then you can do

#define I_HATE_MACROS(a, b) typedef a b;

Or

#define I_HATE_MACROS(a, b) template class a;

Then

#include "DisgustingMacroHackery.h"

Answer 2

This is invalid and rejected by implementations because a typedef name is used in the elaborated type specifier

template class addition;

The following is invalid too, because the Standard says that there must be a simple template id contained in the elaborated type specifier. Comeau online and GCC both accept it, though.

template class addition::binary_operation;

You could apply a pervert workaround though to be fully Standards compliant

template<typename T> using alias = T;
template class alias<multiplication>::binary_operation;

At least I could not find it being invalid anymore on a quick glance over the spec.

Answer 3

I ask my self, why do you actually write a .cpp file as you have templates and they should go either all in the header file or in a seprarate file e.g ".icc", which holds the stuff from the cpp file. I am not sure but tempalates definitions should always NOT be in a compilation unit.

See -> Storing C++ template function definitions in a .CPP file