C++的数组，为什么a[i]=i[a]，求解释

Question

比如声明一个数组
int a[5]={1,2,3,4,5}

这时 a[3] 和 3[a] 的值都是3，求解为什么？

据说是因为i[a]=*(a+i)，但是，为什么是这么规定的？是C++内定的规则，还是因为[ ] 中括号除了定义数组，还有更多的意义？求详细解释，非常感谢。

Answer 1

是的[]有两个用途，数组声明 和 下标操作符，
注意在Array subscripting部分，E1[E2]，只要有一个操作符为指针即可，没有顺序的哦。
实际上在C语言中，Array subscripting是完全可以不要的, 因为
E1[E2]
完全等价于
(*(E1+(E2)))

数组声明
3.5.4.2 Array declarators

Constraints

The expression that specifies the size of an array shall be an
integral constant expression that has a value greater than zero.

Semantics

If, in the declaration `` T D1 ,'' D1 has the form

     D[ constant-expression<opt>]

下标操作符
3.3.2.1 Array subscripting

Constraints

**One of the expressions shall have type ``pointer to object type ,''

the other expression shall have integral type, and the result has type
`` type .''**

Semantics

A postfix expression followed by an expression in square brackets
[] is a subscripted designation of a member of an array object. The
definition of the subscript operator [] is that E1[E2] is identical to
(*(E1+(E2))) . Because of the conversion rules that apply to the
binary + operator, if E1 is an array object (equivalently, a pointer
to the initial member of an array object) and E2 is an integer, E1[E2]
designates the E2 -th member of E1 (counting from zero).