c++ - How does void* work as a universal reference type?

Question

From Programming Language Pragmatics, by Scott

For systems programming, or to facilitate the writing of general-purpose con- tainer (collection) objects (lists, stacks, queues, sets, etc.) that hold references to other objects, several languages provide a universal reference type. In C and C++, this type is called void *. In Clu it is called any; in Modula-2, address; in Modula-3, refany; in Java, Object; in C#, object.

In C and C++, how does void * work as a universal reference type?

void * is always only a pointer type, while a universal reference type contains all values, both pointers and nonpointers. So I can't see how void * is a universal reference type.

Thanks.

Answer 1

A void* pointer will generally hold any pointer that is not a C++ pointer-to-member. It's rather inconvenient in practice, since you need to cast it to another pointer type before you can use it. You also need to convert it to the same pointer type that it was converted from to make the void*, otherwise you risk undefined behavior.

A good example would be the qsort function. It takes a void* pointer as a parameter, meaning it can point to an array of anything. The comparison function you pass to qsort must know how to cast two void* pointers back to the types of the array elements in order to compare them.