c++ tie,ignore

ignore

一个未指定的类型对象，任何值都可以没有影响地赋值给它。通常使用tie来解压一个元组，作为可以忽略的占位符。

#include <iostream>
#include <string>
#include <set>
#include <tuple>
 
int main()
{
    std::ignore = 9;
    std::ignore = "hello";
    std::ignore = std::make_pair<int,std::string>(8, "23");

    std::set<std::string> set_of_str;
    bool inserted = false;
    std::tie(std::ignore, inserted) = set_of_str.insert("Test");
    if (inserted)
    {
        std::cout << "Value was inserted successfully
";
    }

    return 0;
}

tie

创建一个元组的左值引用

template<typename... _Elements>
constexpr tuple<_Elements&...>
tie(_Elements&... __args) noexcept
{
    return tuple<_Elements&...>(__args...);
}

可以看到，tie函数返回的是一个tuple的左值引用

tie函数可以用来解压一个pair,tuple,也可以用来产生一个结构体的字典序比较

#include <iostream>
#include <string>
#include <set>
#include <tuple>
#include <algorithm>
 
struct S {
    int n;
    std::string s;
    float d;

    S(int in, std::string && ss, float fd):n(in),s(ss),d(fd)
    {}

    bool operator<(const S& rhs) const
    {
        // compares n to rhs.n,
        // then s to rhs.s,
        // then d to rhs.d
        return std::tie(n, s, d) < std::tie(rhs.n, rhs.s, rhs.d);
    }
};
 
int main()
{
    std::set<S> set_of_s; // S is LessThanComparable
 
    S value {42, "Test", 3.14};
    std::set<S>::iterator iter;
    bool inserted;
 
    // unpacks the return value of insert into iter and inserted
    std::tie(iter, inserted) = set_of_s.insert(value);
    set_of_s.emplace(12,"t2", 9.8);
 
    if (inserted)
        std::cout << "Value was inserted successfully
";

    for_each(set_of_s.begin(), set_of_s.end(), [](const S & value){
        std::cout << value.n << ' ' << value.s << ' ' << value.d << std::endl;
    });

    return 0;
}

输出：

Value was inserted successfully
12 t2 9.8
42 Test 3.14

经过排序之后，(12,"t2", 9.8) 在前面