std::tie (3) - Linux Manuals

std::tie: std::tie

NAME

std::tie - std::tie

Synopsis

Defined in header <tuple>
template< class... Types > (since C++11)
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

  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:
// 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