Boost程序选项添加选项语法 [英] Boost Program Options Add Options Syntax

问题描述

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

I am writing a program that uses Boost's Program Options library and I noticed the following syntax that has haunted me since I saw it:

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

我看到在标头中，operator()被覆盖了，但是我不确定如何使它在语法上正确.

I see that in the header, operator() is overridden, but I'm not sure how that allows this to be syntactically correct.

第二，与仅多次调用add_options()相比，此语法有什么优势(除了炫耀您可以像这样操作语法的事实)?

Secondly, is there any advantage to this syntax, compared with just calling add_options() multiple times (besides showing off the fact that you can manipulate syntax like this)?

推荐答案

add_options成员函数返回类型为options_description_easy_init的对象.后者的operator()重载以返回对自身的引用.这样，您就可以按照代码段中所示的方式链接呼叫.

The add_options member function returns an object of type options_description_easy_init. The latter has operator() overloaded to return a reference to itself. This allows you to chain the calls as you've shown in the snippet.

多次链接调用和调用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，它都会向所有者添加选项( options_description).如果您多次呼叫add_options，则每次呼叫都会创建一个options_description_easy_init的新实例.

The difference between chaining the calls and calling add_options several times is that in the former case a single instance of options_description_easy_init is created and each time you invoke operator() on it, it adds the options to the owner (options_description). If you were to call add_options multiple times each call would create a new instance of options_description_easy_init.

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