![\"\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2021\/09\/TimelineCpp20Modules.png\")
<\/h2>\nThe Long History of Modules in C++<\/h2>\n![\"wg21](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2020\/05\/wg21-structure-2019-11.png\")
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<\/h2>\nModules are one of the four prominent features of C++20. They overcome the restrictions of header files and promise a lot: faster build-times, fewer violations of the One-Definition-Rule, less usage of the preprocessor. Today, I want to create a simple math module.<\/p>\n
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<\/h2>\n<\/h2>\nModules may be older than you think. My short historic detour should give only an idea of how long it takes to get something such valuable into the C++ standard.<\/p>\n
In 2004, Daveed Vandevoorde wrote the proposal N1736.pdf<\/a>, describing the first-time module idea. It took until 2012 to get a dedicated Study Group (SG2, Modules) for modules. In 2017, Clang 5.0 and MSVC 19.1 provided the first implementation. One year later, the Modules TS (technical specification) was finalized. Around the same time, Google proposed the so-called ATOM (Another Take On Modules) proposal (P0947<\/a>) for modules. In 2019, the Modules TS and the ATOM proposal were merged into the C++20 committee draft (N4842<\/a>), the syntax I present in my posts to modules.<\/p>\n The C++ standardization process is democratic. Section Standardization<\/a> gives you more information about the standard and the standardization process. The image to the right shows the various study groups.<\/p>\n Explaining modules from the user’s perspective is quite easy, but this will not hold for the implementer’s perspective. My plan for this post is to start with simple modules math and add more features to it as we go.<\/p>\n First, here is my first module: The expression export module math<\/span> is the module declaration. By putting export<\/span> before the function adds,<\/span> add <\/span>is exported and can, therefore, be used by a consumer of my module.<\/p>\n import math<\/span> imports the module math<\/span> and makes the exported names in the module visible to the client.cpp<\/span>. Before I build the module, let me say a few words about module declaration files.<\/p>\n Did you notice the strange name of the module: math.ixx<\/span>.<\/p>\n To compile the module, you must use a very current Clang, GCC, or cl.exe compiler. I go into this post with cl.exe on Windows. The Microsoft blog provides two excellent introductions to modules: Overview of modules in C++<\/a> and\u00a0C++ Modules conformance improvements with MSVC in Visual Studio 2019 16.5<\/a>. In contrast, the lack of introductions to the Clang and GCC compilers makes it quite challenging to use modules.<\/p>\n Here are more details about my used Microsoft compiler:<\/p>\n These are the steps to compile and use the module with the Microsoft compiler. I only show the minimal command line. With an older Microsoft compiler, you must use at least \/std:cpplatest.<\/span><\/p>\n For obvious reasons, I will not show you the output of the program execution. Let me change this.<\/p>\n The global module fragment is meant to compose module interfaces. It’s a place to use preprocessor directives such as\u00a0#include<\/span>\u00a0so the module interface can compile. The module interface does not export the code in the global module fragment.<\/p>\n The second version of the module math<\/span> supports the two functions add<\/span> and getProduct<\/span>.<\/p>\n I included the necessary headers between the global module fragment (line 1) and the module declaration (line 2).<\/p>\n The client imports the module math<\/span> and uses its functionality:<\/p>\n Maybe, you don’t like to use a Standard Library Header anymore. Microsoft supports modules for all STL headers. Here is what I have found in the post “Using C++ Modules in Visual Studio 2017<\/a>” from the Microsoft C++ team blog.<\/p>\n To use the Microsoft Standard Library modules, specify the exception handling model (\/EHsc)<\/span><\/a> and the multithreading library (\/MD)<\/span><\/a>.\u00a0 Additionally, you have to use the flag\u00a0\/std:c++latest<\/span><\/a>.<\/p>\n Here are the modified versions of the interface file math2.ixx<\/span> and the source file client2.cpp<\/span>.<\/p>\nThe math<\/span> Modul<\/h2>\n
\n<\/span><\/p>\n\n\/\/ math.ixx<\/span>\n\nexport<\/span> module math;\n\nexport<\/span> int<\/span> add<\/span>(int<\/span> fir, int<\/span> sec){\n return<\/span> fir +<\/span> sec;\n} \n<\/pre>\n<\/div>\n\n\/\/ client.cpp<\/span>\n\nimport math;\n\nint<\/span> main<\/span>() {\n \n add(2000<\/span>, 20<\/span>);\n \n}\n<\/pre>\n<\/div>\nModule Declaration Files<\/h3>\n
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Compile the Module math<\/span><\/h3>\n
![\"MicrosoftCompiler\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2020\/05\/MicrosoftCompiler.png\")
<\/p>\n\ncl.exe \/experimental:module \/c math.ixx \/\/ 1<\/span>\ncl.exe \/experimental:module client.cpp math.obj \/\/ 2<\/span> \n<\/pre>\n<\/div>\n\n
\n<\/span><\/li>\n![\"moduleNotFound1\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2020\/05\/moduleNotFound1.png\")
<\/p>\nGlobal Module Fragment<\/h3>\n
\n\/\/ math1.ixx<\/span>\nmodule; \/\/ global module fragment (1)<\/span>\n\n#include <numeric><\/span>\n#include <vector><\/span>\n\nexport<\/span> module math; \/\/ module declaration (2)<\/span>\n\nexport<\/span> int<\/span> add<\/span>(int<\/span> fir, int<\/span> sec){\n return<\/span> fir +<\/span> sec;\n}\n\nexport<\/span> int<\/span> getProduct<\/span>(const<\/span> std::<\/span>vector<<\/span>int<\/span>>&<\/span> vec) {\n return<\/span> std::<\/span>accumulate(vec.begin(), vec.end(), 1<\/span>, std::<\/span>multiplies<<\/span>int<\/span>><\/span>());\n}\n<\/pre>\n<\/div>\n\n\/\/ client1.cpp<\/span>\n\n#include <iostream><\/span>\n#include <vector><\/span>\n\nimport math;\n\nint<\/span> main<\/span>() {\n \n std::<\/span>cout <<<\/span> std::<\/span>endl; \n \n std::<\/span>cout <<<\/span> \"add(2000, 20): \"<\/span> <<<\/span> add(2000<\/span>, 20<\/span>) <<<\/span> std::<\/span>endl;\n \n std::<\/span>vector<<\/span>int<\/span>><\/span> myVec{1<\/span>, 2<\/span>, 3<\/span>, 4<\/span>, 5<\/span>, 6<\/span>, 7<\/span>, 8<\/span>, 9<\/span>, 10<\/span>};\n \n std::<\/span>cout <<<\/span> \"getProduct(myVec): \"<\/span> <<<\/span> getProduct(myVec) <<<\/span> std::<\/span>endl;\n \n std::<\/span>cout <<<\/span> std::<\/span>endl;\n \n}\n<\/pre>\n<\/div>\n![\"client1\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2020\/05\/client1.png\")
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std.regex<\/code><\/span> provides the content of the header <regex><\/code><\/span><\/li>\nstd.filesystem<\/code><\/span> provides the content of the header <experimental\/filesystem<\/span>><\/code><\/li>\nstd.memory<\/code><\/span> provides the content of the header <memory><\/code><\/span><\/li>\nstd.threading<\/code><\/span> provides the contents of headers <atomic><\/code>, <condition_variable><\/code>, <future><\/code>, <mutex><\/code>, <shared_mutex><\/code>,and <thread><\/code><\/span><\/li>\nstd.core<\/code><\/span> provides everything else in the C++ Standard Library<\/li>\n<\/ul>\n<\/div>\n<\/li>\n<\/ul>\n\n
\n\/\/ math2.ixx<\/span>\nmodule; \n\nimport std.core; \/\/ (1)<\/span>\n\nexport<\/span> module math; \n\nexport<\/span> int<\/span> add<\/span>(int<\/span> fir, int<\/span> sec){\n return<\/span> fir +<\/span> sec;\n}\n\nexport<\/span> int<\/span> getProduct<\/span>(const<\/span> std::<\/span>vector<<\/span>int<\/span>>&<\/span> vec) {\n return<\/span> std::<\/span>accumulate(vec.begin(), vec.end(), 1<\/span>, std::<\/span>multiplies<<\/span>int<\/span>><\/span>());\n}\n<\/pre>\n<\/div>\n\n
\n\/\/ client2.cpp<\/span>\n\nimport std.core; \/\/ (1)<\/span>\n\nimport math;\n\nint<\/span> main<\/span>() {\n \n std::<\/span>cout <<<\/span> std::<\/span>endl; \n \n std::<\/span>cout <<<\/span> \"add(2000, 20): \"<\/span> <<<\/span> add(2000<\/span>, 20<\/span>) <<<\/span> std::<\/span>endl;\n \n std::<\/span>vector<<\/span>int<\/span>><\/span> myVec{1<\/span>, 2<\/span>, 3<\/span>, 4<\/span>,