当 infact 已经转换时取消引用 void 指针错误

Question

Situation

Consider the following source code which aims to print two a's, ie output should be "aa":

#include <stdio.h>

int main ()
{
char a = 'a';
void* a_pnt = &a;
void* b_pnt = (char*)a_pnt;

printf("%c", *(char*)a_pnt);
printf("%c", *b_pnt);// why is the compiler saying I am dereferencing a void pointer? It was already cast
return 0;
}

Complication

The printing of the first "a" works but the compiler is giving a compile time error on line 10 (second printing of "a") saying:

Invalid use of void expression

and a warning on the same line saying:

Dereferencing 'void *' pointer

Although b_pnt was indeed declared a void pointer, it was cast to a character pointer in its definition on line 7. My only guess as to why its complaining is something to do with the fact that I can only cast when referencing at the same time. My hunch is based off the fact that the first variable works just fine.

Solution

The solution is declare and define a character variable called 'b' and cast to character pointer upfront before printing it:

#include <stdio.h>

int main ()
{
char a = 'a';
void* a_pnt = &a;
void* b_pnt = a_pnt;
char b = *((char*)b_pnt);

printf("%c", *(char*)a_pnt);
printf("%c", b);// why is the compiler saying I am dereferencing a pointer?
return 0;
}

The question still remains: Why did the initial attempt fail?

I am deliberately starting off with void pointer to illustrate the issue. It could indeed have been avoided entirely ofcourse with the correct pointer type.

Answer 1

Just because you performed a cast when assigning to b_pnt doesn't mean that it's type changed. It's type is still void * and dereferencing it is an error.

You can freely assign any non-function pointer type to a void * without a warning. But the compiler doesn't keep track of what kind of pointer was stored there, so it's still a void * that needs to be casted before it can be dereferenced.