operator[]= overload?

Question

Okay, I'm trying to make a quick little class to work as a sort of hash table. If I can get it to work then I should be able to do this:

  StringHash* hash = new StringHash;
  hash["test"] = "This is a test";
  printf(hash["test"]);

And it should print out "This is a test".

It looks like I have 2 problems at this point. Firstly I did this:

const char* operator[](const char* key) {
  for(int i = 0; i < hashSize; ++i) {
    if(strcmp(hkeys[i], key) == 0) {return values[i];}
  }
  return NULL;
}

But when I try to look up a value the compiler complains that

error: invalid types `StringHash*[const char[5]]' for array subscript

Secondly operator[]= does not appear to be the correct syntax here. The only other thing I could find was &operator[] but I don't think that will work since I have to code the lookup procedure??? (Isn't that syntax just used to return an array item reference?)

Is what I'm trying to do here even possible? Any advice appreciated. :)

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Seems to be some confusion about what I'm trying to do. I'll post my code:

http://pastebin.com/5Na1Xvaz

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Finished product after all the help:

http://pastebin.com/gx4gnYy8

Answer 1

The error is because hash is a pointer. Change to:

StringHash hash;

Answer 2

The other answers relate to your first question. As for your second...

If you return a reference, then you're returning an lvalue. You can always assign to an lvalue.

Yes, it (pretty much) really is that simple. I recommend reading carefully for whether or not you need const in various places, though.

What I remember reading is that you should provide a const and a non-const overload for operator[], something like so:

MyType const &operator[](int index) const; // This is the array access version (no assignment allowed), which should work on const objects
MyType &operator[](int index);      // This is the array access or assignment version, which is necessarily non-const.

See this link for more information.

Answer 3

hash isn't a StringHash object. Its a pointer to one.

You can do this:

(*hash)["test"] = "This is a test";

Or you can ask yourself why you need a pointer to it in the first place,

StringHash hash;
hash["test" = "This is a test";

... or even if you do, why you wouldn't use a smart pointer like auto_ptr.

#include <memory>
std::auto_ptr<StringHash> hash( new StringHash );
(*hash)["test"] = "This is a test";