Question

我正在编写一个使用Boost程序选项库的程序，我注意到以下语法在我看到之后一直困扰着我：

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

我在标题中看到，operator（）被覆盖了，但是我不确定这是如何允许它在语法上正确的。

其次，与多次调用add_options（）相比，这种语法是否有任何优势（除了展示你可以像这样操作语法这一事实）？

Answer 1

add_options成员函数返回类型为options_description_easy_init的对象。后者重载operator()以返回对自身的引用。这使您可以按照片段中显示的方式链接呼叫。

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，创建了options_description_easy_init的单个实例，并且每次在其上调用operator()时，它都会添加所有者的选项（options_description）。如果您多次致电add_options，则每次通话都会创建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。

比较我的一个家庭项目：

("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.")

到此：

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.");

线长很重要。而且不必在所有事物面前都有visible.add_options()，这样可以更容易阅读。

Boost程序选项添加选项语法

2 个答案: