左值和右值

左值(lvalue)和右值(rvalue)最先来源于编译。在C语言中表示位于赋值运算符两侧的两个值，左边的就叫左值，右边的就叫右值。

它们的定义是：

左值指的是如果一个表达式可以引用到某一个对象，并且这个对象是一块内存空间且可以被检查和存储，那么这个表达式就可以作为一个左值。

右值指的是引用了一个存储在某个内存地址里的数据。

C++ introduces a new mechanism called "rvalue reference" which, among other things, allows us to detect rvalue arguments via function overloading. All we have to do is write a constructor with an rvalue reference parameter. Inside that constructor we can do anything we want with the source, as long as we leave it in some valid state:

    string(string&& that)   // string&& is an rvalue reference to a string    {        data = that.data;        that.data = 0;    }

What have we done here? Instead of deeply copying the heap data, we have just copied the pointer and then set the original pointer to null. In effect, we have "stolen" the data that originally belonged to the source string. Again, the key insight is that under no circumstance could the client detect that the source had been modified. Since we don't really do a copy here, we call this constructor a "move constructor". Its job is to move resources from one object to another instead of copying them.

void stove（double && r3）

stove(double && r3) 直接把 r3的引用据为己有，并且原有的r3的引用失效