std::exchange
Aus cppreference.com
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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 ); |
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template< class T, class U = T > constexpr T exchange( T& obj, U&& new_value ) noexcept(/* see below */); |
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Ersetzt den Wert von obj durch new_value und gibt den alten Wert von obj zurück.
Parameters
| obj | - | Objekt, dessen Wert ersetzt werden soll |
| new_value | - | Wert, der obj zugewiesen werden soll
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| Type requirements | ||
-T must meet the requirements of . Es muß ebenfalls möglich sein, Objekte vom Typ U an Objekte des Typs T zuzuweisen
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Rückgabewerte
der alte Wert von obj
Ausnahmen
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(none) |
(until C++23) | |||||||||
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noexcept specification: (seit C++11)
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(since C++23) |
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;
}
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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
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, ...
See also
tauscht die Werte von zwei Objekten Original: swaps the values of two objects The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (Funktions-Template) | |
(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) |