{"id":5678,"date":"2019-05-05T18:17:00","date_gmt":"2019-05-05T18:17:00","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/c-20-more-details-to-modules\/"},"modified":"2023-06-26T10:11:04","modified_gmt":"2023-06-26T10:11:04","slug":"c-20-more-details-to-modules","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/c-20-more-details-to-modules\/","title":{"rendered":"More Details to Modules"},"content":{"rendered":"

 My last post gave you an introduction to modules in C++20. This post shows how to use existing modules.<\/p>\n

<\/p>\n

\"boxes<\/p>\n

Before I begin this post, let me shortly sum up where we ended in my first post to modules<\/a>.<\/p>\n

<\/span>A Short Recap<\/span><\/h2>\n

I created a module math1<\/span>, which consisted of a module interface unit and a module implementation unit, and a client which used it. Here are the three source files.<\/p>\n

<\/span>Module Interface Unit<\/span><\/h3>\n
\n
\/\/ math1.cppm<\/span>\r\n\r\nexport<\/span> module math1;\r\n\r\nexport<\/span> int<\/span> add<\/span>(int<\/span> fir, int<\/span> sec);\r\n<\/pre>\n<\/div>\n
<\/span>Module Implementation Unit<\/span><\/h3>\n
\n
\/\/ math1.cpp<\/span>\r\n\r\nmodule math1;\r\n\r\nint<\/span> add<\/span>(int<\/span> fir, int<\/span> sec){\r\n    return<\/span> fir +<\/span> sec;\r\n}\r\n<\/pre>\n<\/div>\n
<\/span>Client<\/span><\/h3>\n
\n
\/\/ main1.cpp<\/span>\r\n\r\nimport math1;\r\n\r\nint<\/span> main<\/span>(){\r\n   \r\n   add(2000<\/span>, 20<\/span>);\r\n   \r\n}\r\n<\/pre>\n<\/div>\n
 <\/p>\n
I compiled the program with a current clang and cl.exe compiler. From now on, I will stick with the cl.exe compiler because the compile line is slightly shorter. As promised in my last post, let me show you the program’s output.<\/p>\n<\/p>\n
<\/span>Using a Standard Module<\/span><\/h2>\n
Essentially, neither the module interface nor the implementation unit changed in module math2.<\/p>\n
<\/span>Module Interface Unit<\/span><\/h3>\n
\n
\/\/ math2.cppm<\/span>\r\n\r\nexport<\/span> module math2;\r\n\r\nexport<\/span> int<\/span> add<\/span>(int<\/span> fir, int<\/span> sec);\r\n<\/pre>\n<\/div>\n
<\/span>Module Implementation Unit<\/span><\/h3>\n
\n
\/\/ math2.cpp<\/span>\r\n\r\nmodule math2;\r\n\r\nint<\/span> add<\/span>(int<\/span> fir, int<\/span> sec){\r\n    return<\/span> fir +<\/span> sec;\r\n}\r\n<\/pre>\n<\/div>\n
<\/span>Client<\/span><\/h3>\n
\n
\/\/ main2.cpp<\/span>\r\n\r\n\/\/#include <iostream><\/span>\r\n\r\nimport std.core;\r\n\r\nimport math2;\r\n\r\nint<\/span> main<\/span>(){\r\n\t\r\n    std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n   \r\n    std::<\/span>cout <<<\/span> \"add(2000, 20): \"<\/span> <<<\/span> add(2000<\/span>, 20<\/span>) <<<\/span> std::<\/span>endl;\r\n   \r\n}\r\n<\/pre>\n<\/div>\n
 <\/p>\n
Thanks to the module std.core<\/span>, I can show the result of the addition.<\/p>\n
\"math2\"<\/p>\n
Using the header <iostream><\/span> would also be possible. Of course, I hear your question about which modules are available. Here is what I have from the “Using C++ Modules in Visual Studio 2017” post from the Microsoft C++ team blog.<\/p>\n
<\/span>C++ Modules in Visual Studio 2017<\/span><\/h2>\n
    \n
  • std.regex<\/code><\/span> provides the content of the header <regex><\/code><\/span><\/li>\n
  • std.filesystem<\/code><\/span> provides the content of the header <experimental\/filesystem<\/span>><\/code><\/li>\n
  • std.memory<\/code><\/span> provides the content of the header <memory><\/code><\/span><\/li>\n
  • std.threading<\/code><\/span> provides the contents of headers <atomic><\/code>, <condition_variable><\/code>, <future><\/code>, <mutex><\/code>, <shared_mutex><\/code>, <thread><\/code><\/span><\/li>\n
  • std.core<\/code><\/span> provides everything else in the C++ Standard Library<\/li>\n<\/ul>\n
    Modules provide a higher abstraction than headers. This makes it quite comfortable to use them. Additionally, you can specify which name of a module should be exported or not.<\/p>\n
    <\/span>Export versus Non-Export<\/span><\/h2>\n
    The next module math3<\/span> is a little more complicated than the previous one. Here is the interface.<\/p>\n
    <\/span>Module Interface Unit<\/span><\/h3>\n
     <\/p>\n
    \n
    \/\/ math3.cppm<\/span>\r\n\r\nimport std.core;\r\n\r\nexport<\/span> module math3;\r\n\r\nint<\/span> add<\/span>(int<\/span> fir, int<\/span> sec);\r\n\r\nexport<\/span> int<\/span> mult<\/span>(int<\/span> fir, int<\/span> sec);\r\n\r\nexport<\/span> void<\/span> doTheMath<\/span>();\r\n<\/pre>\n<\/div>\n
     <\/p>\n
    The module interface unit contains the exporting module declaration: export module math3;. <\/span>The module declaration starts the so-called module purview<\/b>. Only names after the module purview declared with export<\/span> are exported. If not, the name is not visible outside the module and has module linkage. This holds in particular for the function add<\/span> but not for the mult<\/span> and doTheMath<\/span>.<\/p>\n
    <\/span>Module Implementation Unit<\/span><\/h3>\n
    \n
    \/\/ math3.cpp<\/span>\r\n\r\nmodule math3;\r\n\r\nint<\/span> add<\/span>(int<\/span> fir, int<\/span> sec){\r\n    return<\/span> fir +<\/span> sec;\r\n}\r\n\r\nint<\/span> mult<\/span>(int<\/span> fir, int<\/span> sec){\r\n    return<\/span> fir *<\/span> sec;\r\n}\r\n\r\nvoid<\/span> doTheMath<\/span>(){\r\n\tstd::<\/span>cout <<<\/span> \"add(2000, 20): \"<\/span> <<<\/span> add(2000<\/span>, 20<\/span>) <<<\/span> std::<\/span>endl;\r\n}\r\n<\/pre>\n<\/div>\n
     <\/p>\n
    There is nothing to add to the module implementation unit. The main <\/span>program is more interesting.<\/p>\n
    <\/span>Client<\/span><\/h3>\n
    \n
    \/\/ main3.cpp<\/span>\r\n\r\n\/\/ #include <iostream>    \/\/ (1)<\/span>\r\n\/\/ #include <numeric>     \/\/ (1)<\/span>\r\n\/\/ #include <string>      \/\/ (1)<\/span>\r\n\/\/ #include <vector>      \/\/ (1)<\/span>\r\nimport std.core;          \/\/ (2)<\/span>\r\n\r\nimport math3;\r\n\r\nint<\/span> main<\/span>(){\r\n\t\r\n    std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n   \r\n    \/\/ std::cout << \"add(2000, 20): \" << add(2000, 20) << std::endl;   \/\/ (3)<\/span>\r\n\t\r\n    std::<\/span>vector<<\/span>int<\/span>><\/span> myVec =<\/span> {1<\/span>, 2<\/span>, 3<\/span>, 4<\/span>, 5<\/span>, 6<\/span>, 7<\/span>, 8<\/span>, 9<\/span>, 10<\/span>};\r\n\t\r\n    std::<\/span>string doc =<\/span> \"std::accumulate(myVec.begin(), myVec.end(), mult): \"<\/span>; \r\n    auto<\/span> prod =<\/span> std::<\/span>accumulate(myVec.begin(), myVec.end(), 1<\/span>, mult);\r\n\t\r\n    std::<\/span>cout <<<\/span> doc <<<\/span> prod <<<\/span> std::<\/span>endl; \r\n\t\r\n    doTheMath();\r\n\t\r\n}\r\n<\/pre>\n<\/div>\n
     <\/p>\n
    You see, modules are pretty comfortable in my case. Instead of using the four headers in the line (1),  I’m okay with a simple import std.core<\/span> in line (2). That was it. Here is the output of the program.<\/p>\n
     \"math3\"<\/p>\n
    Now, to the question: What happens if I add the function add<\/span> in line (3). To recap, add<\/span> is not exported and has module linkage. <\/p>\n
    \"math3Add\"<\/p>\n
    The compiler complains that the function add<\/span> is used in the main program, but the name add<\/span> is not visible.<\/p>\n
    <\/span>Further Details<\/span><\/h2>\n
    First, you can export in various ways.<\/p>\n
    <\/span>Export<\/span><\/h3>\n
    Exporting names with export specifiers, such as in math3.cppm<\/span> is tedious.<\/p>\n
    Export Specifier<\/h4>\n
    \n
    \/\/ math3.cppm<\/span>\r\n\r\nimport std.core;\r\n\r\nexport<\/span> module math3;\r\n\r\nint<\/span> add<\/span>(int<\/span> fir, int<\/span> sec);\r\n\r\nexport<\/span> int<\/span> mult<\/span>(int<\/span> fir, int<\/span> sec);\r\n\r\nexport<\/span> void<\/span> doTheMath<\/span>()<\/pre>\n<\/div>\n
     <\/div>\n
    Instead of an export specifier, you can use an exported group.<\/div>\n
    \n
    Exported Group<\/h4>\n
    \n
    \/\/ math3.cppm<\/span>\r\n\r\nimport std.core;\r\n\r\nexport<\/span> module math3;\r\n\r\nint<\/span> add<\/span>(int<\/span> fir, int<\/span> sec);

    export {<\/span> 
      
        int<\/span> mult<\/span>(int<\/span> fir, int<\/span> sec);\r\n    void<\/span> doTheMath<\/span>();

    }<\/pre>\n<\/div>\n
     <\/div>\n
    The third variation is to use an exported namespace.<\/div>\n
    \n
    Exported Namespace<\/h4>\n<\/div>\n
    \n
    \/\/ math3.cppm<\/span>\r\n\r\nimport std.core;\r\n\r\nexport<\/span> module math3;

    namespace math3<\/span> {

        int<\/span> add<\/span>(int<\/span> fir, int<\/span> sec);

    }

    export namespace math3 {<\/span> 
      
        int<\/span> mult<\/span>(int<\/span> fir, int<\/span> sec);\r\n    void<\/span> doTheMath<\/span>();

    }<\/pre>\n<\/div>\n
     <\/div>\n
    All three variations are semantically equivalent.<\/div>\n
     <\/div>\n
    It may also be quite comfortable to re-export a module<\/p>\n
    Re-Export a Module<\/h4>\n
    Sometimes, you want to export something which you imported from another module. If you don’t export the imported module, the imported module has, consequently, module linkage, and its names are not visible outside the module. Here is a concrete example.<\/p>\n
    Visible versus Invisible<\/h4>\n
    Imagine, I want to import and use the module math.core<\/span> and math.core2<\/span> in a new module math<\/span>. Here is the module interface unit of math.core <\/span>and math.core2.
    <\/span><\/p>\n
      \n
    • Re-exported modules<\/li>\n<\/ul>\n
       <\/p>\n
      \n
      \/\/ module interface unit of math.core<\/span>\r\n\r\nexport<\/span> math.core\r\n\r\nexport<\/span> int<\/span> mult(int<\/span> fir, int<\/span> sec); \r\n<\/pre>\n<\/div>\n
       <\/p>\n<\/div>\n
      \n
      \/\/ module interface unit of math.core2<\/span>\r\n\r\nexport<\/span> math.core2\r\n\r\nexport<\/span> int<\/span> add(int<\/span> fir, int<\/span> sec); \r\n<\/pre>\n<\/div>\n
       <\/p>\n
      Next, here is the new module math<\/span>.<\/p>\n
        \n
      • The new module math<\/span><\/li>\n<\/ul>\n
         <\/p>\n
        \n
        \/\/ module interface unit of math<\/span>\r\n\r\nexport<\/span> module math;\r\n\r\nimport math.core;           \/\/ not exported with mult<\/span>\r\nexport<\/span> import math.core2;   \/\/ exported with add<\/span>\r\n\r\n\r\n\/\/ module implementation unit of math<\/span>\r\n\r\nmult(1100<\/span>, 2<\/span>);             \/\/ fine<\/span>\r\nadd(2000<\/span>, 20<\/span>);             \/\/ fine<\/span>\r\n<\/pre>\n<\/div>\n
         <\/p>\n
        As you can see, it’s fine to use the exported and non-exported names in the module math<\/span>. But the module math.core<\/span> is not exported. Only a client using the module math will see the difference.<\/p>\n
          \n
        • Client<\/li>\n<\/ul>\n
          \n
          \/\/ Client<\/span>\r\n\r\nimport math\r\n\r\nmult<\/span>(1100<\/span>, 2<\/span>);             \/\/ ERROR<\/span>\r\nadd(2000<\/span>, 20<\/span>);             \/\/ fine<\/span>\r\n<\/pre>\n<\/div>\n
           <\/p>\n
          The function mult<\/span> has module linkage and is not visible outside the module. Only the function add<\/span> is visible.<\/p>\n
          Repackage Modules<\/h4>\n
          There is a comfortable way to repackage modules. Just put them in an exported group.<\/p>\n
          \n
          export<\/span> module math;\r\n\r\nexport<\/span>{\r\n\r\n    import math.core;\r\n    import math.core2;\r\n    import math.basics;\r\n\t\r\n}\r\n<\/pre>\n<\/div>\n
           <\/p>\n
          This makes all names visible for a client which imports the module math<\/span>.<\/p>\n
          <\/span>What’s next?<\/span><\/h2>\n
          With my next post, I begin the last main topic of the C++ core guidelines: rules to the standard library. Believe it or not, many professional C++ developers don’t use the standard template library (STL). This holds, in particular, true for the algorithms of the STL.<\/p>\n
           <\/p>\n
           <\/p>\n
           <\/p><\/p>\n","protected":false},"excerpt":{"rendered":"
           My last post gave you an introduction to modules in C++20. This post shows how to use existing modules.<\/p>\n","protected":false},"author":21,"featured_media":5674,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[372],"tags":[443],"class_list":["post-5678","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-modern-c","tag-modules"],"_links":{"self":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/5678","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/users\/21"}],"replies":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/comments?post=5678"}],"version-history":[{"count":1,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/5678\/revisions"}],"predecessor-version":[{"id":6788,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/5678\/revisions\/6788"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media\/5674"}],"wp:attachment":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media?parent=5678"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/categories?post=5678"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/tags?post=5678"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}