How to explicitly instantiate a template for all members of MPL vector in C++?

Question

Consider the following header file:

// Foo.h
class Foo {
    public: 
        template 
        void read(T& value);
};

I

Answer 1

If you intend to use your class only in a single module (i.e. you won't export it) you can use boost/mpl/for_each. The template function defined this way (using mpl/for_each) won't be exported (even if you declare __declspec(export) before class name or function signature):

// Foo.cpp
#include 
#include 

template
void read(T& value)
{
...
}

using types = boost::mpl::vector;

//template instantiation
struct call_read {
  template  
  void operator()(T)
  {
    T t; //You should make sure that T can be created this way
    ((Foo*)nullptr)->read(t); //this line tells to compiler with templates it should instantiate
  }
};

void instantiate()
{
  boost::mpl::for_each(call_read());
}

If you need export/import you structure and template methods there is the solution using boost/preprocessor

// Foo.h
#ifdef 
#    define API __declspec(dllexport)
#else
#    define API __declspec(dllimport)
#endif

class API Foo {
public:
  template void read(T& value);
};

// Foo.cpp
#include 
#include 
#include 

template
void read(T& value)
{
...
}

//using this macro you can define both boost::mpl structure AND instantiate explicitly your template function
#define VARIANT_LIST (std::wstring)(long)(int)
using types = boost::mpl::vector;

//Here we should use our API macro
#define EXPLICIT_INSTANTIATION(r, d, __type__) \
  template API void Foo::read<__type__>(__type__&);
BOOST_PP_SEQ_FOR_EACH(EXPLICIT_INSTANTIATION, _, VARIANT_LIST)

If you don't need this extra functionality the first solution is much cleaner I guess