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std::tie

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< cpp‎ | utility‎ | tuple
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Defined in header <tuple>
template< class... Types >
tuple<Types&...> tie( Types&... args );
(since C++11)
(until C++14)
template< class... Types >
constexpr tuple<Types&...> tie( Types&... args );
(since C++14)

Creates a tuple of lvalue references to its arguments or instances of std::ignore.

Contents

[edit] Parameters

args - zero or more lvalue arguments to construct the tuple from

[edit] Return value

A std::tuple object containing lvalue references.

[edit] Exceptions

noexcept specification:  
noexcept
  

[edit] Example

std::tie can be used to introduce lexicographical comparison to a struct or to unpack a tuple:

#include <iostream>
#include <string>
#include <set>
#include <tuple>
 
struct S {
    int n;
    std::string s;
    float d;
    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);
 
    if (inserted)
        std::cout << "Value was inserted successfully\n";
}

Output:

Value was inserted successfully
creates a tuple object of the type defined by the argument types
(function template) [edit]
creates a tuple of rvalue references
(function template) [edit]
creates a tuple by concatenating any number of tuples
(function template) [edit]
placeholder to skip an element when unpacking a tuple using tie
(constant) [edit]