为什么要输入一个void指针？

Question

Being new to C, the only practical usage I have gotten out of void pointers is for versatile functions that may store different data types in a given pointer. Therefore I did not type-cast my pointer when doing memory allocation.

I have seen some code examples that sometimes use void pointers, but they get type-cast. Why is this useful? Why not directly create desired type of pointer instead of a void?

Answer 1

There are two reasons for casting a void pointer to another type in C.

1. If you want to access something being pointed to by the pointer ( *(int*)p = 42 )
2. If you are actually writing code in the common subset of C and C++, rather than "real" C. See also Do I cast the result of malloc?

The reason for 1 should be obvious. Number two is because C++ disallows the implicit conversion from void* to other types, while C allows it.

Answer 2

You need to cast void pointers to something else if you want to dereference them, for instance you get a void pointer as a function parameter and you know for sure this is an integer:

void some_function(void * some_param) {
    int some_value = *some_param; /* won't work, you can't dereference a void pointer */
}

void some_function(void * some_param) {
    int some_value = *((int *) some_param); /* ok */
}