std::exchange

 

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定义于头文件 <utility>
template< class T, class U = T > T exchange( T& obj, U&& new_value );		(自 C++14 起) (constexpr 自 C++20 起) (conditionally noexcept 自 C++23 起)

用 new_value 替换 obj 的值，并返回 obj 的旧值。

参数

obj	-	要替换其值的对象
new_value	-	要赋值给 obj 的值
类型要求
- T 必须满足 MoveConstructible 的要求。此外，必须可以将 U 类型的对象移动赋值给 T 类型的对象。

返回值

obj 的旧值。

异常

(无)	(直到 C++23)
noexcept 规范：   noexcept(     std::is_nothrow_move_constructible_v<T> &&     std::is_nothrow_assignable_v<T&, U> )	(自 C++23 起)

可能的实现

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; }

注解

当实现移动构造函数时，以及对于不需要特殊清理的成员，移动赋值运算符，可以使用 std::exchange

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;
    }
};

特性测试 宏	值	Std	特性
__cpp_lib_exchange_function	201304L	(C++14)	std::exchange

示例

运行此代码

#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";
}

输出

0
0
12
 
1, 2, 3, 4,
 
f()
 
Fibonacci sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

参见

swap	交换两个对象的值 (函数模板)
atomic_exchangeatomic_exchange_explicit (C++11)(C++11)	原子地用非原子参数替换原子对象的值，并返回原子的旧值 (函数模板)