Question

This code:

int p = 10;
void *q;
*q = 10;

Does not compile:

‘=’ : cannot convert from ‘int’ to ‘void *’

However, this code compiles fine:

int p = 10;
void *q;
q = &p;

What is the reason behind it?

Answer 1

A void * points to data of an unknown type (if it is initialized, which yours is not).

You can only assign to variables of a known type, or via pointers of a known type.

int p = 10;
void *q = &p;

*(int *)q = 20;

if (p != 20)
    ...something has gone horribly wrong...

This converts the void * into an int * and then assigns a value to that dereferenced integer pointer.