Abdullah  Kozey

Abdullah Kozey


Boost.Hana: Your Standard Library for Metaprogramming For C++


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. The documentation covers everything you should need including installing the library, a tutorial explaining what Hana is and how to use it, and an extensive reference section with examples. The remainder of this README is mostly for people that wish to work on the library itself, not for its users.

An offline copy of the documentation can be obtained 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 is 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 include directory contains the library itself, which is header only.
  • The test directory contains the source code for all the unit tests.


Please see CONTRIBUTING.md.


Please see LICENSE.md.


To release a new version of Hana, use the util/release.sh script. The script will merge develop to master, create a tag on master and then bump the version on develop. The tag on master will be annotated with the contents of the RELEASE_NOTES.md file. Once the release.sh script has been run, the master and develop branches should be pushed manually, as well as the tag that was created on master. Finally, create a GitHub release pointing to the new tag on master.

Author: boostorg
Source code: https://github.com/boostorg/hana
License: BSL-1.0 license


What is GEEK

Buddha Community

Boost.Hana: Your Standard Library for Metaprogramming For C++
Tamale  Moses

Tamale Moses


How to Run C/C++ in Sublime Text?

C and C++ are the most powerful programming language in the world. Most of the super fast and complex libraries and algorithms are written in C or C++. Most powerful Kernel programs are also written in C. So, there is no way to skip it.

In programming competitions, most programmers prefer to write code in C or C++. Tourist is considered the worlds top programming contestant of all ages who write code in C++.

During programming competitions, programmers prefer to use a lightweight editor to focus on coding and algorithm designing. VimSublime Text, and Notepad++ are the most common editors for us. Apart from the competition, many software developers and professionals love to use Sublime Text just because of its flexibility.

I have discussed the steps we need to complete in this blog post before running a C/C++ code in Sublime Text. We will take the inputs from an input file and print outputs to an output file without using freopen file related functions in C/C++.

#cpp #c #c-programming #sublimetext #c++ #c/c++

Dicey Issues in C/C++

If you are familiar with C/C++then you must have come across some unusual things and if you haven’t, then you are about to. The below codes are checked twice before adding, so feel free to share this article with your friends. The following displays some of the issues:

  1. Using multiple variables in the print function
  2. Comparing Signed integer with unsigned integer
  3. Putting a semicolon at the end of the loop statement
  4. C preprocessor doesn’t need a semicolon
  5. Size of the string matters
  6. Macros and equations aren’t good friends
  7. Never compare Floating data type with double data type
  8. Arrays have a boundary
  9. Character constants are different from string literals
  10. Difference between single(=) and double(==) equal signs.

The below code generates no error since a print function can take any number of inputs but creates a mismatch with the variables. The print function is used to display characters, strings, integers, float, octal, and hexadecimal values onto the output screen. The format specifier is used to display the value of a variable.

  1. %d indicates Integer Format Specifier
  2. %f indicates Float Format Specifier
  3. %c indicates Character Format Specifier
  4. %s indicates String Format Specifier
  5. %u indicates Unsigned Integer Format Specifier
  6. %ld indicates Long Int Format Specifier

Image for post

A signed integer is a 32-bit datum that encodes an integer in the range [-2147483648 to 2147483647]. An unsigned integer is a 32-bit datum that encodes a non-negative integer in the range [0 to 4294967295]. The signed integer is represented in twos-complement notation. In the below code the signed integer will be converted to the maximum unsigned integer then compared with the unsigned integer.

Image for post

#problems-with-c #dicey-issues-in-c #c-programming #c++ #c #cplusplus

Abdullah  Kozey

Abdullah Kozey


Boost.GIL: Generic Image Library | Requires C++14 Since Boost 1.80


DocumentationGitHub ActionsAppVeyorAzure PipelinesRegressionCodecov
developGitHub ActionsAppVeyorAzuregilcodecov
masterGitHub ActionsAppVeyorAzuregilcodecov



Boost.GIL is a part of the Boost C++ Libraries.

The Boost Generic Image Library (GIL) is a C++14 header-only library that abstracts image representations from algorithms and allows writing code that can work on a variety of images with performance similar to hand-writing for a specific image type.


See RELEASES.md for release notes.

See CONTRIBUTING.md for instructions about how to build and run tests and examples using Boost.Build or CMake.

See example/README.md for GIL usage examples.

See example/b2/README.md for Boost.Build configuration examples.

See example/cmake/README.md for CMake configuration examples.


The Boost Generic Image Library (GIL) requires:

  • C++14 compiler (GCC 6, clang 3.9, MSVC++ 14.1 (1910) or any later version)
  • Boost header-only libraries

Optionally, in order to build and run tests and examples:

  • Boost.Filesystem
  • Boost.Test
  • Headers and libraries of libjpeg, libpng, libtiff, libraw for the I/O extension and some of examples.


The official repository contains the following branches:

master This holds the most recent snapshot with code that is known to be stable.

develop This holds the most recent snapshot. It may contain unstable code.


There is number of communication channels to ask questions and discuss Boost.GIL issues:

Contributing (We Need Your Help!)

If you would like to contribute to Boost.GIL, help us improve the library and maintain high quality, there is number of ways to do it.

If you would like to test the library, contribute new feature or a bug fix, see the CONTRIBUTING.md where the whole development infrastructure and the contributing workflow is explained in details.

You may consider performing code reviews on active pull requests or help with solving reported issues, especially those labelled with:

Any feedback from users and developers, even simple questions about how things work or why they were done a certain way, carries value and can be used to improve the library.


Distributed under the Boost Software License, Version 1.0.

Author:  boostorg
Source code: https://github.com/boostorg/gil
License: BSL-1.0 license


Shaylee  Lemke

Shaylee Lemke


How to solve the implicitly declaring library function warning in C

Learn how to solve the implicitly declaring library function warning in C

#c #c# #c++ #programming-c

Shaylee  Lemke

Shaylee Lemke


C# Source Generators to boost performance in .NET 5

A new C# compiler feature that inspects code and generates additional source files promises to improve performance in a number of scenarios.

Microsoft has introduced a preview of a C# compiler capability called Source Generators that can inspect a program and generate source files that can be added to a compilation. Microsoft says Source Generators can improve performance in a number of scenarios.

Introduced April 29, a Source Generator is a piece of code (a .NET Standard 2.0 assembly) that runs during compilation and can inspect a program to produce additional files that are compiled together with the rest of the code.

#c #c# #c++ #programming-c