我们常说的左值lvalue和右值rvalue并不简单地意味着左右区别，它的原始定义如下：
Definition:
C and C++ have the notion of lvalues and rvalues associated with
variables and constants. The rvalue is the data value of the variable,
that is, what information it contains. The "r" in rvalue can be thought of as "read"
value
. A variable also has an associated lvalue. The "l" in lvalue can be though of as location
,
meaning that a variable has a location that data or information can be
put into. This is contrasted with a constant. A constant has some data
value, that is an rvalue. But, it cannot be written to. It does not
have an lvalue.
Another view of these terms is that objects with an rvalue, namely a
variable or a constant can appear on the right hand side of a
statement. They have some data value that can be manipulated. Only
objects with an lvalue, such as variable, can appear on the left hand
side of a statement. An object must be addressable to store a value.
Here are two examples.
int x;
x = 5; // This is fine, 5 is an rvalue, x can be an lvalue.
5 = x; // This is illegal. A literal constant such as 5 is not
// addressable. It cannot be a lvalue.
引文中说: "The "r" in rvalue can be thought of as "read" value."
就是你可以把 "r" 理解为 "read". 并没有说就是 "read" 的意思!
lvalue, rvalue称为"左值", "右值" 并没有违背原意. 因为,
到目前为止, 所有计算机语言都是将被赋值量置于赋值号左端的, 因此这种称谓和理解非常直观的. 对于赋值量来说, 也是相同的道理.
之所以有"location"和"read"的说法, 是因为在C/C++中, 有很多表达式是表达可赋值单元的,
我们不能简单地理解"lvalue"就是变量. 如: a[i], *p, *(a->p+1), 等等. 这些都是C/C++的表达式,
不是变量, 故用"location"的含义可以避免很多误解. 请看下面的例子:
const int x;
x = 1; // 这里 x 是 rvalue! 所以, 这是错误的赋值
