std::tie

 

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std::tuple

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tuple::tuple

tuple::operator=

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make_tuple

tie

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operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

swap(std::tuple)

get(std::tuple)

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ignore

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Defined in header <tuple>
template<class... Types> std::tuple<Types&...> tie( Types&... args)noexcept;		(since C++11) (constexpr since C++14)

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

Parameters

args

-

zero or more lvalue arguments to construct the tuple from.

Return value

A std::tuple object containing lvalue references.

Possible implementation

template<typename... Args>constexpr// since C++14std::tuple<Args&...> tie(Args&... args)noexcept{return{args...};}

Notes

std::tie may be used to unpack a std::pair because std::tuple has a converting assignment from pairs:

bool result; std::tie(std::ignore, result)= set.insert(value);

Example

1) std::tie can be used to introduce lexicographical comparison to a struct or to unpack a tuple;
2) std::tie can work with structured bindings:

Run this code

#include <cassert>#include <iostream>#include <set>#include <string>#include <tuple>   struct S {int n;std::string s;float d;   friendbool operator<(const S& lhs, const S& rhs)noexcept{// compares lhs.n to rhs.n,// then lhs.s to rhs.s,// then lhs.d to rhs.d// in that order, first non-equal result is returned// or false if all elements are equalreturn std::tie(lhs.n, lhs.s, lhs.d)< std::tie(rhs.n, rhs.s, rhs.d);}};   int main(){// Lexicographical comparison demo:std::set<S> set_of_s;   S value{42, "Test", 3.14};std::set<S>::iterator iter;bool is_inserted;   // Unpack a pair: std::tie(iter, is_inserted)= set_of_s.insert(value);assert(is_inserted);     // std::tie and structured bindings:auto position =[](int w){returnstd::tuple(1* w, 2* w);};   auto[x, y]= position(1);assert(x ==1&& y ==2); std::tie(x, y)= position(2);// reuse x, y with tieassert(x ==2&& y ==4);     // Implicit conversions are permitted:std::tuple<char, short> coordinates(6, 9); std::tie(x, y)= coordinates;assert(x ==6&& y ==9);   // Skip an element:std::string z; std::tie(x, std::ignore, z)=std::tuple(1, 2.0, "Test");assert(x ==1&& z =="Test");}

See also

Structured binding(C++17)	binds the specified names to sub-objects or tuple elements of the initializer
make_tuple (C++11)	creates a tuple object of the type defined by the argument types (function template)
forward_as_tuple (C++11)	creates a tuple of forwarding references (function template)
tuple_cat (C++11)	creates a tuple by concatenating any number of tuples (function template)
ignore (C++11)	placeholder to skip an element when unpacking a tuple using tie (constant)