Boost 程序选项添加选项语法

Question

我正在编写一个使用 Boost 程序选项库的程序，我注意到以下语法自从我看到它以来一直困扰着我：

desc.add_options()
        ("help","produce help message")
        ( /* other flag, value, description pairs here */)
;

我看到在标题中，operator() 被覆盖，但我不确定这如何使它在语法上是正确的。

其次，与只调用 add_options() 多次相比，这种语法有什么优势（除了炫耀您可以像这样操作语法之外）？

Answer 1

add_options 成员函数返回options_description_easy_init 类型的对象。后者重载了operator() 以返回对自身的引用。这允许您像在 sn-p 中显示的那样链接调用。

链接调用和多次调用add_options 之间的区别在于，在前一种情况下，会创建一个options_description_easy_init 的单个实例，并且每次调用operator() 时，它都会将选项添加到所有者（ options_description)。如果您要多次调用add_options，则每次调用都会创建一个options_description_easy_init 的新实例。

Answer 2

优势问题是主观的，但在这种情况下它很简洁。

与我的一个家庭项目进行比较：

("help,h", "Generate this help message")
("output-file,o", po::value<std::string>(), "Output filename. Required.")
("tangent,t", "Generate/load tangent-space basis.")
("collada-output,c", "Write a Collada file, rather than our mesh XML format.")
("arrays,a", "Write arrays instead of indexed verts. Cannot combine with Collada writing.")
("flip-tangent,f", "Change the tangent-space basis matrix's handedness. Negates bitangent.")
("map", po::value<std::string>(), "Map filename. Defaults to the ColladaConv directory's 'stdmap.txt' file.")
("vao", po::value<std::vector<std::string> >(), "Sequence of mappings, of the form:\n"
        "Name # # # #\n"
        "\n"
        "Each # is an attribute index to use for this VAO.\n"
        "Each VAO name must be unique; you cannot use the same VAO in the same place.")

到这里：

visible.add_options()("help,h", "Generate this help message")
visible.add_options()("output-file,o", po::value<std::string>(), "Output filename. Required.")
visible.add_options()("tangent,t", "Generate/load tangent-space basis.");
visible.add_options()("collada-output,c", "Write a Collada file, rather than our mesh XML format.");
visible.add_options()("arrays,a", "Write arrays instead of indexed verts. Cannot combine with Collada writing.");
visible.add_options()("flip-tangent,f", "Change the tangent-space basis matrix's handedness. Negates bitangent.");
visible.add_options()("map", po::value<std::string>(), "Map filename. Defaults to the ColladaConv directory's 'stdmap.txt' file.");
visible.add_options()("vao", po::value<std::vector<std::string> >(), "Sequence of mappings, of the form:\n"
        "Name # # # #\n"
        "\n"
        "Each # is an attribute index to use for this VAO.\n"
        "Each VAO name must be unique; you cannot use the same VAO in the same place.");

行长很重要。并且不必在所有内容前面都有visible.add_options()，这样更容易阅读。