std::exchange - cppreference.com (original) (raw)

| | | | | ----------------------------------------------------------------------- | | ------------------------------------------------------------------------- | | 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.

Contents

• 1 Parameters
• 2 Return value
• 3 Exceptions
• 4 Possible implementation
• 5 Notes
• 6 Example
• 7 See also

[edit] Parameters

[edit] Return value

The old value of obj.

[edit] Exceptions

[edit] Possible implementation

[edit] 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; } };

[edit] Example

#include #include #include #include   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 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{1, 2, 3, 4});   std::exchange(v, {1, 2, 3, 4});   std::copy(begin(v), end(v), std::ostream_iterator(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, ...

[edit] See also