NULL pointer and void pointer?

Are they the same thing? If not, what is the difference? Thanks for your input.

Nope...they are different....one is basically describing the content the pointer is actually pointing to while the other is simply a pointer of type 'void'...in other words

Code:

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void* void_pointer;
int* null_pointer = 0;

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Thus...a null-pointer can be a 'void'-pointer as well...

Code:

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void* void_and_null_pointer = 0;

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Quote:

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Originally Posted by dullboy

Are they the same thing? If not, what is the difference? Thanks for your input.

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void is used to mean "Nothing", as in void foo() which means foo is a function that returns "Nothing".

void* is used to mean "something that can be anything", as in void *foo() which means foo is a function that can return a pointer to "anything"... cool heh :-)

NULL means "A thing which is Nothing" as in int *p = NULL; which means p is a pointer to int and it currently points to a thing which is defined as nothing :-)

A NULL is simply defined as 0 in most C/C++ implementations.

So, it is common to see code like if(!foo(...)) instead of if(NULL == foo(...)) which is also correct....

All the above mentioned definitions are mine and mine alone and do not represent the feelings of the C++ community in any way... :-)

Excellent! Thanks.

Quote:

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Originally Posted by Andreas Masur

Nope...they are different....one is basically describing the content the pointer is actually pointing to while the other is simply a pointer of type 'void'...in other words

Code:

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void* void_pointer;
int* null_pointer = 0;

---

Thus...a null-pointer can be a 'void'-pointer as well...

Code:

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void* void_and_null_pointer = 0;

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Quote:

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Originally Posted by raghuvamshi

void* is used to mean "something that can be anything", as in void *foo() which means foo is a function that can return a pointer to "anything"... cool heh :-)

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No, not cool: "I have a pointer to... something. Now what?"

void* is only really useful if a function manipulates a pointer as a value in itself, rather than as a "reference" to something else, e.g. ostream operator<<(ostream, void*).

void * is also commonly used in standard APIs. For example, a thread function takes a void * parameter, a generic way of being able to pass it data.

This is how C APIs are often done, but in C++ APIs it is better to define a base class and expect a pointer to the base class.

Threads, for example, can be done like this:

Code:

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// in header:

class ThreadClass
{
public:
  virtual ThreadClass* threadFunc() = 0;
};

thread_t create_thread( ThreadClass * pThreadClass );

/// in .cxx file

namespace
{
  extern "C" void * the_one_thread_function( void * param )
  {
      ThreadClass * pThreadClass( reinterpret_cast< ThreadClass * >(param ) );
      return pThreadClass->threadFunc();
  }
}

thread_t create_thread( ThreadClass * pThreadClass )
{
  thread_t result;
  pthread_create( &result, 0, the_one_thread_function, 0 );
  return result;
}

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This is a very loose description to show how one might start writing a C++ wrapper to pthread_create which removes the need for void *. (The return value of the thread function is also void *, and you might want to use a different class, a ThreadResult class).

(If you want to implement this in full, then it is probably a good idea to take an optional thread-attribute parameter (2nd parameter to pthread_create). You'd also need some kind of collection (somewhere to store the ThreadFunc object and subsequently be able to delete it), thus a thread pool ).