空指针的目的是什么？

Question

Why are void pointers necessary, as long as one could cast any pointer type to any pointer type, ie:

char b = 5; 
int*a = (int*)&b;//both upcasting

or

int b = 10;
char*a = (char*)b;//and downcasting are allowed

?

Also, why there is no need for cast when using malloc/calloc/realloc?

Answer 1

one could cast any pointer type to any pointer type

Not quite. void * is defined to convert any object pointer ¹ to void * and back again with an equivalent value. It does not need any cast.

In less common architectures, the size and range of some other pointers may be smaller than void * . Casting between other pointers type may lose necessary information.

void * provides a universal object pointer type.

void *p = any_object_pointer; // No casts required
any_object_pointer = p;       // No casts required

char * could substitute for void * , except conversion to and from other object pointers requires casts.

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OP's char b = 5; int*a = (int*)&b; char b = 5; int*a = (int*)&b; risks undefined behavior as the alignment needs of int * may exceed char * .

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¹ Function pointers may be wider than void* . void * and other pointers are object pointers . C lacks a truly universal pointer type.

Answer 2

void pointers are really useful to create a generic API. You might think of qsort function which can be used to sort arrays of any types. void pointers can be used if the API does not know the concrete type of the pointer.

void qsort(
   void *base,
   size_t number,
   size_t width,
   int (__cdecl *compare )(const void *, const void *)
);

Regarding allocation functions, it's the same thing. The C runtime does not know the type of the effective object. But this is not a problem as user can use generic pointer void .

So void pointers are considered as generic pointers, very useful for polymorphism, that's why the C language makes casting to void optional.

Answer 3

"Why are void pointers necessary, as long as one could cast any pointer type".

The alloc function would then have to use common denominator such as char. But then it would be confusing to have to cast to whatever we really need. "void" just means "a bunch of bytes".