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std::tie

From cppreference.com
< cpp‎ | utility‎ | tuple
 
 
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Defined in header <tuple>
template< class... Types >
std::tuple<Types&...> tie( Types&... args ) noexcept;
(since C++11)
(until C++14)
template< class... Types >
constexpr std::tuple<Types&...> tie( Types&... args ) noexcept;
(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] Possible implementation

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

[edit] 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);

[edit] 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:

#include <cassert>
#include <iostream>
#include <set>
#include <string>
#include <tuple>
 
struct S
{
    int n;
    std::string s;
    float d;
 
    friend bool 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 equal
        return 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) { return std::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 tie
    assert(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);
}

[edit] See also

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