Do Stuff Inside a Function That is Declared Constexpr

specialize_constexpr.cpp

#include<iostream>

struct tag1 { };
struct tag2 { };
struct tag3 { };
struct tag4 { };

template <typename Tag>
struct foo_impl { };

template <typename Tag>
constexpr auto foo(Tag)
{
  return foo_impl<Tag>::apply();
};

template <>
struct foo_impl<tag1>
{
  static constexpr int apply()
  {
    return 5;
  }
};

template <>
struct foo_impl<tag2>
{
  static int apply()
  {
    std::cout << "Do something not constexpr friendly.\n";
    return 5;
  }
};

template <>
struct foo_impl<tag3>
{
  static int apply()
  {
    return ([](int x) { return x; })(5);
  }
};

template <>
struct foo_impl<tag4>
{
  static int apply()
  {
    std::aligned_storage<sizeof(int)> storage;
    new (&storage) int(5);
    int x = *reinterpret_cast<int*>(&storage);
    return x;
  }
};

int main()
{
  constexpr int x1 = foo(tag1{});
  int x2 = foo(tag2{});
  int x3 = foo(tag3{});
  int x4 = foo(tag4{});
}