std::exchange (3) - Linux Man Pages

std::exchange: std::exchange

NAME

std::exchange - std::exchange

Synopsis


Defined in header <utility>
template< class T, class U = T > (since C++14)
T exchange( T& obj, U&& new_value ); (until C++20)
template< class T, class U = T > (since C++20)
constexpr T exchange( T& obj, U&& new_value );


Replaces the value of obj with new_value and returns the old value of obj.

Parameters


obj - object whose value to replace
new_value - the value to assign to obj

Type requirements


-
T must meet the requirements of MoveConstructible. Also, it must be possible to move-assign objects of type U to objects of type T

Return value


The old value of obj

Exceptions


(none)

Possible implementation


  template<class T, class U = T>
  T exchange(T& obj, U&& new_value)
  {
      T old_value = std::move(obj);
      obj = std::forward<U>(new_value);
      return old_value;
  }

Notes


This function can be used when implementing move_assignment_operators and move_constructors:


  struct S
  {
    int* p;
    int n;


    S(S&& other)
      :p{std::exchange(other.p, nullptr)}
      ,n{std::exchange(other.n, 0)}
    {}


    S& operator=(S&& other) {
      p = std::exchange(other.p, nullptr); // move p, while leaving nullptr in other.p
      n = std::exchange(other.n, 0); // move n, while leaving zero in other.n
      return *this;
    }
  };

Example


// Run this code


  #include <iostream>
  #include <utility>
  #include <vector>
  #include <iterator>


  class stream
  {
    public:


     using flags_type = int;


    public:


      flags_type flags() const
      { return flags_; }


      ///Replaces flags_ by newf, and returns the old value.
      flags_type flags(flags_type newf)
      { return std::exchange(flags_, newf); }


    private:


      flags_type flags_ = 0;
  };


  void f() { std::cout << "f()"; }


  int main()
  {
     stream s;


     std::cout << s.flags() << '\n';
     std::cout << s.flags(12) << '\n';
     std::cout << s.flags() << "\n\n";


     std::vector<int> v;


     //Since the second template parameter has a default value, it is possible
     //to use a braced-init-list as second argument. The expression below
     //is equivalent to std::exchange(v, std::vector<int>{1,2,3,4});


     std::exchange(v, {1,2,3,4});


     std::copy(begin(v),end(v), std::ostream_iterator<int>(std::cout,", "));


     std::cout << "\n\n";


     void (*fun)();


     //the default value of template parameter also makes possible to use a
     //normal function as second argument. The expression below is equivalent to
     //std::exchange(fun, static_cast<void(*)()>(f))
     std::exchange(fun,f);
     fun();
  }

Output:


  0
  0
  12


  1, 2, 3, 4,


  f()

See also


                         swaps the values of two objects
swap (function template)


atomic_exchange
atomic_exchange_explicit atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic
                         (function template)
(C++11)
(C++11)