左值和右值

Lvalues and Rvalues

左值和右值

 

We'll have more to say about expressions in Chapter 5, but for now it is useful to know that there are two kinds of expressions in C++:

我们在第五章再详细探讨表达式，现在先介绍 C++ 的两种表达式：

 

lvalue (pronounced "ell-value"): An expression that is an lvalue may appear as either the left-hand or right-hand side of an assignment.

左值（发音为 ell-value）：左值可以出现在赋值语句的左边或右边。

 

rvalue (pronounced "are-value"): An expression that is an rvalue may appear on the right- but not left-hand side of an assignment.

右值（发音为 are-value）：右值只能出现在赋值的右边，不能出现在赋值语句的左边。

 

Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned. Given the variables:

变量是左值，因此可以出现在赋值语句的左边。数字字面值是右值，因此不能被赋值。给定以下变量：

 

int units_sold = 0;

double sales_price = 0, total_revenue = 0;

 

it is a compile-time error to write either of the following:

下列两条语句都会产生编译错误：

 

// error: arithmetic expression is not an lvalue units_sold * sales_price = total_revenue;

// error: literal constant is not an lvalue 0 = 1;

 

Some operators, such as assignment, require that one of their operands be an lvalue. As a result, lvalues can be used in more contexts than can rvalues. The context in which an lvalue appears determines how it is used. For example, in the expression

有些操作符，比如赋值，要求其中的一个操作数必须是左值。结果，可以使用左值的上下文比右值更广。左值出现的上下文决定了左值是如何使用的。例如，表达式

units_sold = units_sold + 1;

 

the variable units_sold is used as the operand to two different operators. The + operator cares only about the values of its operands. The value of a variable is the value currently stored in the memory associated with that variable. The effect of the addition is to fetch that value and add one to it.

中，units_sold 变量被用作两种不同操作符的操作数。+ 操作符仅关心其操作数的值。变量的值是当前存储在和该变量相关联的内存中的值。加法操作符的作用是取得变量的值并加 1。

 

The variable units_sold is also used as the left-hand side of

可以看出，左值和右值是跟左和右挂钩的。

 

另外：http://blog.youkuaiyun.com/lovewwy/archive/2009/02/26/3940670.aspx