3rd Attempt at C++ q

main_a.cpp

#include "./templates_a.h"

int main()
{
    LocalTypeA bbb;
    bbb.internal_ = new LocalTypeB();
    do_foo<LocalTypeA>(LocalTypeA());
    return 0;
}

main_b.cpp

#include "./templates_b.h"

int main()
{
    do_foo<double>(0.0);
    do_foo<LocalTypeB>(LocalTypeB());
    return 0;
}

templates_a.h

#include "./templates_top.h"
#include "./templates_b.h"

class LocalTypeA { 
  public:
    LocalTypeB* internal_;
};

template<typename T>
std::enable_if_t<std::is_same<T, LocalTypeA>::value, int> do_foo(T bb)
{
    // Some detailed business logic 
    // specific to LocalTypeA and LocalTypeB that ideally isn't in a header
    return do_foo<LocalTypeB>(*(bb.internal_));
}

templates_b.h

#ifndef HEADER_B
#define HEADER_B

#include <type_traits>
#include "templates_top.h"

class LocalTypeB { 
    public: 
        bool the_val = false;
};

template<typename T>
std::enable_if_t<std::is_same<T, LocalTypeB>::value, int> do_foo(T bb)
{
    // Some detailed business logic here 
    // specific to LocalTypeB that ideally isn't in a header
    // Also - we need to invoke generic impl do_foo
    return do_foo<bool>(bb.the_val);
}

#endif

templates_top.h

#ifndef TOPHEADER
#define TOPHEADER

#include <type_traits>

template<typename T>
std::enable_if_t<std::is_fundamental<T>::value, int> do_foo(T bb)
{
  // Some generic business logic for all fundamental types here
    return 0;
}

#endif