左值、右值、可修改左值、不可修改左值

Assignment: l-values and r-values

Some languages use the idea of l-values and r-values. Lvalues are values that have addresses being programmatically accessible to the running program (e.g., via some address-of–operator like "&" in C/C++), meaning that they are variables or dereferenced references to a certain memory location. Rvalues can be lvalues (see below.) or non-lvalues—a term only used to distinguish from lvalues. Consider the C expression (4 + 9). When executed, the computer generates an integer value of 13, but because the program has not explicitly designated where in the computer this 13 is stored, the expression is an rvalue. On the other hand, if a C program declares a variable x and assigns the value of 13 to x, then the expression (x) has a value of 13 and is an lvalue.

In C, the term lvalue originally meant something that could be assigned to (coming from left-value, indicating it was on the left side of the assignment operator), but since 'const' was added to the language, this now is termed a 'modifiable lvalue'.由const关键字修饰的左值属于不可修改的左值，其值不可修改，
eg： 辨别常指针与常量的指针:
char * const p；
char const * p；
const char *p；
上述三个有什么区别？
char* const p; //p为只读指针。
char const *p；//(*p)值只读的指针。
const char *p； //和char const *p意思相同。

数组名是不可修改的左值
注意：有一样操作只能在指针里进行而无法在数组中进行，那就是修改它的值。数组名是不可修改的左值，它的值是不能改变的。