Must the pointer be initialized before use , then how to understand char * p?

Question

new learner ; something puzzle about pointer;

As I learn from books, before using the pointer it must be initialized , so we usually use like this

int a = 12;

int * p = &a; 

so I understand why int* p = 12 is wrong ,because it has no address;

then I find something today while coding , That is from this :

char * months[12] = {"Jan", "Feb", "Mar", "April", "May" , "Jun", "Jul"    
,"Aug","Sep","Oct","Nov","Dec"};

Then another usually used situation came to my mind , That is :

char *p = "string"; (this is ok , why int * a = 12 can't be allowed ?)

I am puzzled. when is it initialized and how ? and why int * a = 12 can't be auto initialized ? maybe something about the arrange of memory.

Answer 1

First off:

int a = 12;
int* p = &a;

This works because &a is a memory address.

int* p = 12;

This fails mostly because 12 is not a memory address. It's also true that 12, by itself, has no address, but this would be better reflected by a snippet like int* p = &12; (which wouldn't work, as you correctly noted).

An interesting property of pointers is that they are often used to designate the start of a list of values. For instance, take this array of integers:

int a[] = {1, 3, 7, 13};

It can trivially be turned into an integer pointer.

int* p = a; // magic!

The pointee is the first element of a, so *p == 1. Now, you can also do p[0] (which is 1, too), p[1] == 3, p[3] == 7, and p[4] == 13.

The reason char* foo = "bar" works is that "bar" is not a single value: it's a character array in disguise. Single characters are denoted by single quotes. As a matter of fact:

"bar"[0] == 'b'
"bar"[1] == 'a'
"bar"[2] == 'r'

The compiler has special support for string literals (quoted strings) that make it possible to assign them straight to pointers. For instance, char* foo = "bar" is valid.

A C99-compliant compiler also has support for array literals. For instance, int* p = (int [3]){1, 2, 3}; is valid. The character array and the int array will be given a global address, because the people who made C felt that it was a useful thing to do.

Answer 2

int* p = 12 is wrong because the assigned value may or may not belongs to memory address. You are forcing p to point at that location.
char *p = "string" is allowed because compiler already has set the space for the string and p is pointing to the first character of that string.