请解释这个使用std :: ignore的代码 [英] Please explain this code that uses std::ignore

问题描述

我正在阅读有关 std :: ignore 从cppreference。我发现很难理解这个对象的真正目的，而且示例代码并没有这么正义。例如，在下面的代码中，如何和为什么插入设置为true？对我来说没有什么意义。

  #include< iostream> 
＃include< string> 
 #include< set> 
 #include< tuple> 
 
 int main（）
 {
 std :: set< std :: string> set_of_str; 
 bool inserted; 
 std :: tie（std :: ignore，inserted）= set_of_str.insert（Test）; 
 if（inserted）{
 std :: cout<< 值已成功插入\\\
; 
} 
} 
  

如果有人可以向我解释代码，谢谢。感谢。

解决方案

set :: insert 返回一个对，其中第一个是迭代器到插入的元素，第二个是一个bool，说明元素是否被插入。 p>

std :: tie 创建一个左值引用的元组。当分配给 insert 的结果时，它允许您将 tie 中的变量设置为插入的结果成员 / p>

std :: ignore

因此，基本上，这个代码忽略了元素的迭代器。 插入并插入 到第二 set :: insert 返回的对的成员，表示是否插入了一个元素。




I'm reading the documentation on std::ignore from cppreference. I find it quite hard to grasp the true purpose of this object, and the example code doesn't do it much justice. For example, in the below code, how and why is inserted set to true? It doesn't make much sense to me.

#include <iostream>
#include <string>
#include <set>
#include <tuple>

int main()
{
    std::set<std::string> set_of_str;
    bool inserted;
    std::tie(std::ignore, inserted) = set_of_str.insert("Test");
    if (inserted) {
        std::cout << "Value was inserted sucessfully\n";
    }
}

If someone can explain the code to me, it would be appreciated. Thanks.

解决方案

set::insert returns a pair where first is the iterator to the inserted element and second is a bool saying whether the element was inserted.

std::tie creates a tuple of lvalue references. When assigned to the result from insert it enables you to set the variables in the tie to the results of the insert in the return pair's first and second members.

std::ignore is a value that can be assigned to with no effect.

So basically, this code ignores the iterator to the element where "Test" was inserted and asigns inserted to the second member of the pair returned by set::insert that indicates whether the an element was inserted.

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