app.cpp

#include "lib.h"
#include "common.h"

#include <iostream>

void thing()
{
  auto lib_id1 = get_next_lib_id_int32();
  auto lib_id2 = get_next_lib_id_size();

  auto this_id1 = get_next_id<std::size_t>();
  auto this_id2 = get_next_id<std::int32_t>();

  std::cout <<
    "lib id1: " << lib_id1 << "\n" <<
    "lib id2: " << lib_id2 << "\n" <<
    "this id1: " << this_id1 << "\n" <<
    "this id2: " << this_id2 << "\n";
}

int main()
{
  thing();
  thing();
}

common.h

#include <cstddef>
#include <iostream>

std::size_t do_thing()
{
  static std::size_t id;

  std::cout << "whoa, do_thing has been called!\n";

  return id++;
}

template <typename T>
extern std::size_t get_next_id()
{
  static std::size_t id = do_thing();

  return id;
}

lib.cpp

#include "common.h"
#include "lib.h"

std::size_t get_next_lib_id_int32()
{
  return get_next_id<std::int32_t>();
}

std::size_t get_next_lib_id_size()
{
  return get_next_id<std::size_t>();
}

lib.h

#include <cstddef>

std::size_t get_next_lib_id_int32();
std::size_t get_next_lib_id_size();