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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 ofobj withnew_value and returns the old value ofobj.
Contents |
| obj | - | object whose value to replace |
| new_value | - | the value to assign toobj |
| Type requirements | ||
-T must meet the requirements ofMoveConstructible. Also, it must be possible to move-assign objects of typeU to objects of typeT. | ||
The old value ofobj.
(none) | (until C++23) |
noexcept specification: noexcept( std::is_nothrow_move_constructible_v<T>&& | (since C++23) |
template<class T,class U= T>constexpr// Since C++20T exchange(T& obj, U&& new_value)noexcept(// Since C++23std::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;} |
std::exchange can be used when implementingmove constructors and, for the members that don't requirespecial 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 |
#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:
0012 1, 2, 3, 4, f() Fibonacci sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
| swaps the values of two objects (function template)[edit] | |
(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)[edit] |
