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std::exchange

Aus cppreference.com
< cpp‎ | utility
 
C++
 
Utilities-Bibliothek
 

definiert in Header <utility>
template< class T, class U = T >
T exchange( T& obj, U&& new_value );
 (seit C++14)
template< class T, class U = T >
constexpr T exchange( T& obj, U&& new_value );
template< class T, class U = T >
constexpr T exchange( T& obj, U&& new_value ) noexcept(/* see below */);

Ersetzt den Wert von obj durch new_value und gibt den alten Wert von obj zurück.

Inhaltsverzeichnis

[Bearbeiten] Parameters

obj - Objekt, dessen Wert ersetzt werden soll
new_value - Wert, der obj zugewiesen werden soll
Type requirements
-
T must meet the requirements of . Es muß ebenfalls möglich sein, Objekte vom Typ U an Objekte des Typs T zuzuweisen

[Bearbeiten] Rückgabewerte

der alte Wert von obj

[Bearbeiten] Ausnahmen

(none)

(until C++23)
noexcept specification:  (seit C++11)
(since C++23)

[Bearbeiten] Mögliche Implementierungen 

template<class T, class U = T>
constexpr // since C++20
T exchange(T& obj, U&& new_value)
    noexcept( // since C++23
        std::is_nothrow_move_constructible<T>::value &&
        std::is_nothrow_assignable<T&, U>::value
    )
{
    T old_value = std::move(obj);
    obj = std::forward<U>(new_value);
    return old_value;
}

[Bearbeiten] Anmerkungen

std::exchange kann benutzt werden , um den Verschiebungszuweiseoperatoren und Verschiebekonstuktoren zu implementieren: 

struct S
{
  int n;
 
  S(S&& other) noexcept : n{std::exchange(other.n, 0)}
  {}
 
  S& operator=(S&& other) noexcept 
  {
    if(this != &other)
        n = std::exchange(other.n, 0); // move n, while leaving zero in other.n
    return *this;
  }
};

Feature testing macro: __cpp_lib_exchange_function

[Bearbeiten] Beispiele 

#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();
 
   std::cout << "\n\nFibonacci sequence: ";
   for (int a{0}, b{1}; a < 100; a = std::exchange(b, a + b))
       std::cout << a << ", ";
   std::cout << "...\n";
}

Output: 

0
0
12
 
1, 2, 3, 4, 
 
f()
 
Fibonacci sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

[Bearbeiten] See also
tauscht die Werte von zwei Objekten
Original:
swaps the values of two objects
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(Funktions-Template) [edit]
(C++11)
(C++11)
atomar ersetzt den Wert des atomaren Objekt mit nicht-elementare Argument und gibt den alten Wert des atomaren
Original:
atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic
The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions.

(Funktions-Template) [edit]
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