std::exchange
From cppreference.com
| Defined in header <utility>
|
||
template< class T, class U = T >
T exchange( T& obj, U&& new_value );
|
(since C++14) (constexpr since C++20) (conditionally noexcept since C++23) |
|
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) |
(until C++23) |
|
noexcept specification:
noexcept( std::is_nothrow_move_constructible_v<T> && std::is_nothrow_assignable_v<T&, U> ) |
(since C++23) |
Possible implementation
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;
}
|
Notes
std::exchange can be used when implementing move constructors and, for the members that don't require special cleanup, move assignment operators:
struct S
{
int n;
S(S&& other) noexcept : n{std::exchange(other.n, 0)} {}
S& operator=(S&& other) noexcept
{
n = std::exchange(other.n, 0); // Move n, while leaving zero in other.n
// Note: in case of self-move-assignment, n is unchanged
// Also note: if n is an opaque resource handle that requires
// special cleanup, the resource is leaked.
return *this;
}
};
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_exchange_function |
201304L |
(C++14) | std::exchange
|
Example
Run this code
#include <iostream>
#include <iterator>
#include <utility>
#include <vector>
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
| swaps the values of two objects (function template) | |
(C++11)(C++11) |
atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic (function template) |
