C++笔记：左值和右值

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这两个概念看似简单，其实大部分人很多时候都没有细分他们的差别。最近看到primer书里讨论rvalue reference（右值的引用）时觉得自己应该再回去看看。而事实上左值和右值本身也确实很简单。

这是微软在VS2017中对左右值的定义：https://msdn.microsoft.com/en-us/library/f90831hc.aspx；
Every C++ expression is either an lvalue or an rvalue. An lvalue refers to an object that persists beyond a single expression. You can think of an lvalue as an object that has a name. All variables, including nonmodifiable (const) variables, are lvalues. An rvalue is a temporary value that does not persist beyond the expression that uses it. To better understand the difference between lvalues and rvalues, consider the following example:

在《C++ Primer》第五版中，对左右值的说明如下：

Generally speaking, an lvalue expression refers to an object’s identity whereas an rvalue expression refers to an object’s value.

其实早先lvalue就是如字面指“可以用在赋值符号左边的值”，直到ISO C加入了“const”关键字。所以人们发现并不是所有左值都可以被赋值，所以定义了modifiable lvalues。在C99中对modifiable lvalues的定义为：

an lvalue that does not have array type, does not have an incomplete type, does not have a const-qualified type, and if it is a structure or union, does not have any member (including, recursively, any member or element of all contained aggregates or unions) with a const-qualified type.

左值可以随时转化为右值，然而右值一般不可以转变为左值，除了一些特殊情况，例如：解引用*操作符

int arr[] = {1, 2};int* p = &arr[0];*(p + 1) = 10;   // OK: p + 1 is an rvalue, but *(p + 1) is an lvalue

在c++标准中对lvalue-to-rvalue conversions的描述为：

An lvalue (3.10) of a non-function, non-array type T can be converted to an rvalue. […] If T is a non-class type, the type of the rvalue is the cv-unqualified version of T. Otherwise, the type of the rvalue is T.

CV-qualifier is a term used to describe const and volatile type qualifiers.

Rvalue References（C++11）

rvalue references（右值引用）用&&表示。它与左值引用&不同，不能互相引用。

int i=42;int &r=i;               //ok:r refers to iint &&rr=i;             //error:cannot bind an rvalue reference to an lvalueint &r2=i*42;           //error:i*42 is an rvalueconst int &r3=i*42;     //ok:we can bind a reference to const to an rvalueint &&rr2=i*42;          //ok:bind rr2 to the result of the multiplication

对于产生右值的表达式，we can bind either an lvalue reference to const or an rvalue reference to such expressions（怕翻译过来引起误会就引用primer上的原文）。
可以将右值引用继续赋值给一个左值引用而非右值引用，因为这时右值引用本身已经是一个变量，即左值

int &&r=5;int &&r1=r;     //errorint &r2=r;      //ok

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