如何重载 std::swap()

2021-12-08 00:00:00 performance optimization c++ stl c++-faq

std::swap() 被许多 std 容器(例如 std::list 和 std::vector)使用排序甚至分配.

std::swap() is used by many std containers (such as std::list and std::vector) during sorting and even assignment.

但是 swap() 的 std 实现非常通用，对于自定义类型来说效率很低.

But the std implementation of swap() is very generalized and rather inefficient for custom types.

因此可以通过使用自定义类型特定实现重载 std::swap() 来提高效率.但是如何实现它才能被 std 容器使用?

Thus efficiency can be gained by overloading std::swap() with a custom type specific implementation. But how can you implement it so it will be used by the std containers?

推荐答案

重载 std::swap 的实现(也就是专门化它)的正确方法是将它写在同一个命名空间中作为您要交换的内容，以便可以通过 参数相关查找 (ADL) 找到它).一件特别容易的事情是:

The right way to overload std::swap's implemention (aka specializing it), is to write it in the same namespace as what you're swapping, so that it can be found via argument-dependent lookup (ADL). One particularly easy thing to do is:

class X
{
    // ...
    friend void swap(X& a, X& b)
    {
        using std::swap; // bring in swap for built-in types

        swap(a.base1, b.base1);
        swap(a.base2, b.base2);
        // ...
        swap(a.member1, b.member1);
        swap(a.member2, b.member2);
        // ...
    }
};

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