std::tie

 

C++

Language

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Strings library

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Algorithms library

Iterators library

Numerics library

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Regular expressions library (C++11)

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Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

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Date and time

Function objects

initializer_list (C++11)

bitset

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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

Pairs and tuples

pair

tuple (C++11)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

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Type operations

declval (C++11)

as_const (C++17)

 

std::tuple

Member functions

tuple::tuple

tuple::operator=

tuple::swap

Non-member functions

make_tuple

tie

forward_as_tuple

tuple_cat

operator==operator!=operator<operator<=operator>operator>=

std::swap

get

Helper classes

tuple_size

tuple_element

uses_allocator

ignore

 

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.

Parameters

args

-

zero or more lvalue arguments to construct the tuple from

Return value

A std::tuple object containing lvalue references.

Exceptions

noexcept specification:  

noexcept

  

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

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

Run this code

#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

make_tuple	creates a tuple object of the type defined by the argument types (function template)
forward_as_tuple	creates a tuple of rvalue references (function template)
tuple_cat	creates a tuple by concatenating any number of tuples (function template)
ignore	placeholder to skip an element when unpacking a tuple using tie (constant)