std::tie - cppreference.com (original) (raw)
| | | | | --------------------------------------------------------------------------------------------------- | | ------------------------------------- | | template< class... Types > std::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.
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
std::tiecan be used to introduce lexicographical comparison to a struct or to unpack a tuple;std::tiecan work with structured bindings:
#include
#include
#include
#include
#include
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 set_of_s;
S value{42, "Test", 3.14};
std::set::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);
// Skip an element:
std::string z;
std::tie(x, std::ignore, z) = std::tuple(1, 2.0, "Test");
assert(x == 1 && z == "Test");
}