| Defined in header
<utility> | ||
|---|---|---|
template< class T, class U = T > T exchange( T& obj, U&& new_value ); | (since C++14) |
Replaces the value of obj with new_value and returns the old value of obj.
| 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 |
||
The old value of obj.
(none).
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;
} |
This function can be used when implementing move assignment operators:
struct S
{
int* p;
int n;
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;
}
}#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, std::static_cast<void(*)()>(f))
std::exchange(fun,f);
fun();
}Output:
0 0 12 1, 2, 3, 4, f()
| 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) |
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