NAME¶

std::tie - std::tie

Synopsis¶

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

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);
}

See also¶

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