generic pointer to char conversion problem

compiler is complaining for this string hash function I found online:

error: invalid conversion from 'void*' to 'unsigned char*'

Code:

---

//FNV string hash fcn (src:http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx)
unsigned fnv_hash ( void *key, int len )//Q: how do I use generic ptr key?
{
  unsigned char* p = key;
  unsigned h = 2166136261U;
  int i;
 
  for ( i = 0; i < len; i++ )
    h = ( h * 16777619 ) ^ p[i];
 
  return h;
}

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If you are using a C++ compiler you will probably need to cast your void* to the proper type.

Jim

It's fixed w/ casting as you mentioned:

Code:

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unsigned char* p = (unsigned char*)key;

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BUT shouldnt' it just be

Code:

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unsigned char* p = (unsigned char)key;

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which doesn't compile. I ask b/c in function parameters, we have declared key to be a void pointer, so if it's already a pointer, I'd think the (unsigned char*)key would be dereferencing a ptr...

we have declared key to be a void pointer, so if it's already a pointer, I'd think the (unsigned char*)key would be dereferencing a ptr[/quote]
The syntax that you're using is:

Code:

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x = (type_of_x)y;

---

Hence, there is no dereferencing. You are type casting. Therefore, you should be casting to unsigned char*, not unsigned char.

EDIT:
Then again, in this case consider using static_cast, e.g.,

Code:

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unsigned char* p = static_cast<unsigned char*>(key);

---

I have not learnt about static_cast, but for the type casting format, it's still confusing... But if that's how the compilers roll, I'll go w/ it... I hope you see where my logic is.

But, you know, a cast merely changes the type of something. It never dereferences something.
Therefore, if you do
unsigned char* p = (unsigned char)key;
You are telling the compiler to take the pointer, and convert it (truncate it) to fit an unsigned char. Doesn't make sense.
unsigned char* p = (unsigned char*)key;
This on the other hand tells the compiler to change key into a unsigned char*, which makes sense because we want it to be a pointer to char.
Re not having learned static_cast: google it.