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Your standard library for metaprogramming

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Your standard library for metaprogramming


#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() {
  // Sequences capable of holding heterogeneous objects, and algorithms
  // to manipulate them.
  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"));

  // No compile-time information is lost: even if `animals` can't be a
  // constant expression because it contains strings, its length is constexpr.
  static_assert(hana::length(animals) == 3u, "");

  // Computations on types can be performed with the same syntax as that of
  // normal C++. Believe it or not, everything is done at compile-time.
  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*>), "");

  // And many other goodies to make your life easier, including:
  // 1. Access to elements in a tuple with a sane syntax.
  static_assert(animal_ptrs[0_c] == hana::type_c<Fish*>, "");
  static_assert(animal_ptrs[1_c] == hana::type_c<Dog*>, "");

  // 2. Unroll loops at compile-time without hassle.
  std::string s;
  hana::int_c<10>.times([&]{ s += "x"; });
  // equivalent to s += "x"; s += "x"; ... s += "x";

  // 3. Easily check whether an expression is valid.
  //    This is usually achieved with complex SFINAE-based tricks.
  auto has_name = hana::is_valid([](auto&& x) -> decltype((void)x.name) { });
  static_assert(has_name(animals[0_c]), "");
  static_assert(!has_name(1), "");


You can browse the documentation online at http://boostorg.github.io/hana. You can also get an offline version of the documentation by checking out the gh-pages branch. To avoid overwriting the current directory, you can clone the gh-pages branch into a subdirectory like doc/html:

git clone http://github.com/boostorg/hana --branch=gh-pages --depth=1 doc/html

After issuing this, doc/html will contain exactly the same static website that’s available online. Note that doc/html is automatically ignored by Git so updating the documentation won’t pollute your index.

Hacking on Hana

Setting yourself up to work on Hana is easy. First, you will need an installation of CMake. Once this is done, you can cd to the root of the project and setup the build directory:

mkdir build
cd build
cmake ..

Usually, you’ll want to specify a custom compiler because the system’s compiler is too old:

cmake .. -DCMAKE_CXX_COMPILER=/path/to/compiler

Usually, this will work just fine. However, on some systems, the standard library and/or compiler provided by default does not support C++14. If this is your case, the wiki has more information about setting you up on different systems.

Normally, Hana tries to find Boost headers if you have them on your system. It’s also fine if you don’t have them; a few tests requiring the Boost headers will be disabled in that case. However, if you’d like Hana to use a custom installation of Boost, you can specify the path to this custom installation:

cmake .. -DCMAKE_CXX_COMPILER=/path/to/compiler -DBOOST_ROOT=/path/to/boost

You can now build and run the unit tests and the examples:

cmake --build . --target check

You should be aware that compiling the unit tests is pretty time and RAM consuming, especially the tests for external adapters. This is due to the fact that Hana’s unit tests are very thorough, and also that heterogeneous sequences in other libraries tend to have horrible compile-time performance.

There are also optional targets which are enabled only when the required software is available on your computer. For example, generating the documentation requires Doxygen to be installed. An informative message will be printed during the CMake generation step whenever an optional target is disabled. You can install any missing software and then re-run the CMake generation to update the list of available targets.


You can use the help target to get a list of all the available targets.

If you want to add unit tests or examples, just add a source file in test/ or example/ and then re-run the CMake generation step so the new source file is known to the build system. Let’s suppose the relative path from the root of the project to the new source file is path/to/file.cpp. When you re-run the CMake generation step, a new target named path.to.file will be created, and a test of the same name will also be created. Hence,

cd build # Go back to the build directory
cmake --build . --target path.to.file # Builds the program associated to path/to/file.cpp
ctest -R path.to.file # Runs the program as a test

Tip for Sublime Text users

If you use the provided hana.sublime-project file, you can select the “[Hana] Build current file” build system. When viewing a file to which a target is associated (like a test or an example), you can then compile it by pressing ⌘B, or compile and then run it using ⇧⌘B.

Project organization

The project is organized in a couple of subdirectories. - The benchmark directory contains compile-time and runtime benchmarks to make sure the library is as fast as advertised. The benchmark code is written mostly in the form of eRuby templates. The templates are used to generate C++ files which are then compiled while gathering compilation and execution statistics. - The cmake directory contains various CMake modules and other scripts needed by the build system. - The doc directory contains configuration files needed to generate the documentation. The doc/html subdirectory is automatically ignored by Git; you can conveniently store a local copy of the documentation by cloning the gh-pages branch into that directory, as explained above. - The example directory contains the source code for all the examples of both the tutorial and the reference documentation. - The experimental directory contains various experiments that might or might not make it into Hana at some point. - The include directory contains the library itself, which is header only. - The test directory contains the source code for all the unit tests.

Related material

  • Talk on metaprogramming and Hana at CppCon 2015 (slides/video)
  • Talk on metaprogramming and Hana at C++Now 2015 (slides)
  • Talk on Hana at CppCon 2014 (slides/video)
  • The MPL11 library, which is how Hana started out
  • Talk on the MPL11 at C++Now 2014 (slides/video)
  • Louis Dionne’s bachelor’s thesis was a formalization of C++ metaprogramming through category theory. The thesis is available here, and the slides of a related presentation are available here. Unfortunately, both are in french only.


Please see CONTRIBUTING.md.


Please see LICENSE.md.


This section acts as a reminder of the few simple steps required to release a new version of the library. This is only relevant to Hana’s developers. To release a new version of the library, create an annotated tag using git tag -a. Then, push the tag and create a new GitHub release pointing to that tag. Once that is done, bump the version number in include/boost/hana/version.hpp so that it matches the next planned release.

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