Boost.Hana: compile-time if demo

main.cpp

// Boost.Hana solution
// issue: http://baptiste-wicht.com/posts/2015/07/simulate-static_if-with-c11c14.html
// docs w/ explanation: http://ldionne.com/hana/#tutorial-introspection-is_valid

#include <iostream>
#include <string>

#include <boost/hana.hpp>

auto has_pop_back = boost::hana::is_valid([](auto && obj) -> decltype(obj.pop_back()) { });

template<typename T>
void decrement_kindof(T & value)
{
  boost::hana::if_
  (
    has_pop_back(value),
    [](auto & value) { return value.pop_back(); },
    [](auto & value) { return --value; }
  )(value);
}

int main()
{
  std::string s{"abc"};
  decrement_kindof(s);
  std::cout << s << '\n';

  int i = 5;
  decrement_kindof(i);
  std::cout << i << '\n';
}

output.txt

$ ./static_if.exe
ab
4