6. const usage, combination of first and second level pointers

C file ：

It's all printed out 30,const The amount of modification is called a constant ！ Cannot be an array parameter ！

C++ file ：

C++ in , All appear const Where constant names are , Are replaced by the initialization of constants ！（ Refer to detai C++ Anatomy column , It involves the concept of constant table ！） Design to constants at compile time a Is replaced by a constant ！

const Combined with one or two level pointers
sonst The common mistake in the amount of decoration is ：

1. Constants can no longer be used as lvalues
2. The address of a constant cannot be disclosed to an ordinary pointer or ordinary reference variable （ You can modify the value of a constant indirectly ！）

C++ language norm ：const It's the type closest to him ！

first line ：*p Cannot be used as an lvalue ：*p=20 error ,p=&b Sure （const Modification is int type ）
The second line ： Same as the first line , Modification is int
The third line ： Modification is int * , in other words p Is a constant , Cannot point to other memory ！*p Assignable （ Modify the value of the memory pointed to through pointer dereference ）
In the fourth row ： first const modification int（*p）, The second modification int*（p）！

const Sum up the formula ：
const If there is no pointer on the right * Words ,const It's a non participating type ！

The above is wrong , Will report a mistake ！