這兩個概念看似簡單，其實大部分人很多時候都沒有細分他們的差別。最近看到primer書裏討論rvalue reference（右值的引用）時覺得自己應該再回去看看。而事實上左值和右值本身也確實很簡單。
這是微軟在VS2017中對左右值的定義：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，對左右值的說明如下：
Generally speaking, an lvalue expression refers to an object’s identity whereas an rvalue expression refers to an object’s value.
其實早先lvalue就是如字面指“可以用在賦值符號左邊的值”，直到ISO C加入了“const”關鍵字。所以人們發現並不是所有左值都可以被賦值，所以定義了modifiable lvalues。在C99中對modifiable lvalues的定義爲：
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.
左值可以隨時轉化爲右值，然而右值一般不可以轉變爲左值，除了一些特殊情況，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++標準中對lvalue-to-rvalue conversions的描述爲：
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。它與左值引用&不同，不能互相引用。
int i=42;
int &r=i; //ok:r refers to i
int &&rr=i; //error:cannot bind an rvalue reference to an lvalue
int &r2=i*42; //error:i*42 is an rvalue
const int &r3=i*42; //ok:we can bind a reference to const to an rvalue
int &&rr2=i*42; //ok:bind rr2 to the result of the multiplication對於產生右值的表達式，we can bind either an lvalue reference to const or an rvalue reference to such expressions（怕翻譯過來引起誤會就引用primer上的原文）。
可以將右值引用繼續賦值給一個左值引用而非右值引用，因爲這時右值引用本身已經是一個變量，即左值
int &&r=5;
int &&r1=r; //error
int &r2=r; //ok