NAME

std::tie - std::tie

Synopsis

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

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

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

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)