std::exchange (3) - Linux Manuals

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()