Question

Is void* necessary apart from memory allocation related stuff in C++?
Can you give me an example?

Answer 1

Logging memory addresses

If you want to output a pointer using iostreams (e.g. for logging) then going via void* is the only way of ensuring operator<< hasn’t been overloaded in some crazy way.

#include <iostream>

struct foo {
};

std::ostream& operator<<(std::ostream& out, foo*) {
  return out<<"it's a trap!";
}

int main() {
  foo bar;
  foo *ptr = &bar;

  std::cout << ptr << std::endl;
  std::cout << static_cast<void*>(ptr) << std::endl;
}

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Testing iostream status

iostreams overload operator void* as a status check so that syntax like if (stream) or while (stream) is a short hand way of testing the stream status.

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Template meta programming

You might want to use void* with template metaprogramming sometimes as a reduced catch all, e.g. with SFINAE tricks, but more often than not there’s a nicer way around it using a partial specialisation of one form or another.

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Accessing most derived pointer

As Alf pointed out in the comments dynamic_cast<void*> is also useful for getting at the most derived type in a heirarchy, e.g.:

#include <iostream>

struct other {
  virtual void func() = 0;
  int c;
};

struct foo {
  virtual void func() { std::cout << "foo" << std::endl; }
  int a;
};

struct bar : foo, other {
  virtual void func() { std::cout << "bar" << std::endl; }
  int b;
};

namespace {
  void f(foo *ptr) {
    ptr->func();
    std::cout << ptr << std::endl;
    std::cout << dynamic_cast<void*>(ptr) << std::endl;
  }

  void g(other *ptr) {
    ptr->func();
    std::cout << ptr << std::endl;
    std::cout << dynamic_cast<void*>(ptr) << std::endl;
  }
}

int main() {
  foo a;
  bar b;
  f(&a);
  f(&b);
  g(&b);
}

Gives:

foo
0xbfb815f8
0xbfb815f8
bar
0xbfb815e4
0xbfb815e4
bar
0xbfb815ec
0xbfb815e4

On my system.

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Exceptions

§ 15.3.1 states:

The exception-declaration shall not denote a pointer or reference to
an incomplete type, other than void*, const void*, volatile void*, or
const volatile void*.

So it seems to be the only legal way of catching a pointer to an incomplete type is via void*. (Although I think there’s possibly bigger issues if you actually needed to use that)

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Legacy C uses

There are a lot of “legacy” C uses for void* for storing pointers to data without knowing what it is, but in new C++ code there is almost always a better way of expressing the same functionality.