升壓程序選項添加選項語法

Question

我寫一個使用Boost的程序選項庫的程序，我注意到下面的語法，因爲我看到它已經困擾我：

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

我看到，在標題中，運營商（）被覆蓋，但我不確定這是如何讓它在語法上是正確的。

其次，與多次調用add_options（）相比，這個語法是否有優勢（除了展示你可以像這樣操作語法的事實）？

Answer 1

add_options成員函數返回options_description_easy_init類型的對象。後者有operator()重載以返回對自身的引用。這允許您按照您在代碼段中所示的方式鏈接呼叫。

鏈接呼叫和呼叫add_options幾次的區別是，在每次創建的options_description_easy_init單個實例在前一種情況下，你調用它operator()，其添加的選項所有者（options_description）。如果您要多次撥打add_options，每次呼叫都會創建一個options_description_easy_init的新實例。

Answer 2

優勢問題是主觀的，但在這種情況下它是簡潔的。

比較這離我家項目之一：

("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.") 

這樣：

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."); 

線長度的問題。而且不必在所有內容前都有visible.add_options()，這使得它更易於閱讀。