std::tie

Defined in header `<tuple>`
template< class... Types > 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

namespace detail {
struct ignore_t {
    template <typename T>
    const ignore_t& operator=(const T&) const { return *this; }
};
}
const detail::ignore_t ignore;
 
template <typename... Args>
auto tie(Args&... args) {
    return std::tuple<Args&...>(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:

#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)

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