const& 表示，这个重载只会用于 const、non-const 和 lvalue 对象.

const& means, that this overload will be used only for const, non-const and lvalue object.

const A a = A();
*a;

& 表示，这个重载将只用于非常量对象.

& means, that this overload will be used only for non-const object.

A a;
*a;

&& 表示，这个重载将只用于右值对象.

&& means, that this overload will be used only for rvalue object.

*A();

有关 C++11 标准的此功能的更多信息，您可以阅读这篇文章 什么是*this"的右值引用?

for more information about this feature of C++11 standard you can read this post What is "rvalue reference for *this"?