Boost.Hana Documentation

Boost.Hana is a C++14 library designed for metaprogramming, providing a 'standard library' for compile-time computations. Key features include:

• Heterogeneous Sequences: Sequences (like hana::tuple) that can hold different types of objects, along with algorithms to manipulate them (e.g., hana::transform, hana::reverse, hana::filter).
• Compile-time Metadata: Even if a sequence contains runtime-only data (like std::string), its properties like hana::length remain constexpr.
• Type-level Computations: Perform operations on types using the same syntax as normal C++ (e.g., using hana::type_c and hana::traits).
• Compile-time Loops: Unroll loops at compile-time using hana::int_c<N>.times([&]{ ... });.
• Expression Validation: Easily check if an expression is valid using hana::is_valid (a cleaner alternative to complex SFINAE tricks).

#include <boost/hana.hpp>
#include <cassert>
#include <string>
namespace hana = boost::hana;
using namespace hana::literals;

struct Fish { std::string name; };
struct Cat  { std::string name; };
struct Dog  { std::string name; };

int main() {
  // Heterogeneous sequences and algorithms
  auto animals = hana::make_tuple(Fish{"Nemo"}, Cat{"Garfield"}, Dog{"Snoopy"});
  auto names = hana::transform(animals, [](auto a) {
    return a.name;
  });
  assert(hana::reverse(names) == hana::make_tuple("Snoopy", "Garfield", "Nemo"));

  // Compile-time length
  static_assert(hana::length(animals) == 3u, "");

  // Type-level computations
  auto animal_types = hana::make_tuple(hana::type_c<Fish*>, hana::type_c<Cat&>, hana::type_c<Dog*>);
  auto animal_ptrs = hana::filter(animal_types, [](auto a) {
    return hana::traits::is_pointer(a);
  });
  static_assert(animal_ptrs == hana::make_tuple(hana::type_c<Fish*>, hana::type_c<Dog*>), "");

  // Compile-time loop unrolling
  std::string s;
  hana::int_c<10>.times([&]{ s += "x"; });

  // Expression validation
  auto has_name = hana::is_valid([](auto&& x) -> decltype((void)x.name) { });
  static_assert(has_name(animals[0_c]), "");
  static_assert(!has_name(1), "");
}