return 2d array from function in C++

Question

I have a function declared like so:

unsigned char** Classifier::classify(){
      //...
    unsigned char **chars = new unsigned char *[H];
for(int i = 0; i < H; i++)
    chars[i] = new unsigned char[W*3];

//...

return &chars;
//note: when this is "return chars;" I get the following:  cannot convert ‘unsigned char*’ to ‘unsigned char**’ in return

This is giving me the warning:

Classifier.cpp: In member function ‘unsigned char** Classifier::classify()’:
Classifier.cpp:124: warning: address of local variable ‘chars’ returned

Is this ok to ignore? Basically, my question is how do you return a reference to an array that is defined in the function?

I want to be able to do

unsigned char** someData = classify();

Answer 1

Just return the array, not its address:

return chars;

&chars is a pointer to a pointer to a pointer, but chars is a pointer to a pointer (what you want). Also note that chars is not an array. Pointers and arrays are not the same thing, although they are often confused.

Answer 2

This is never okay to ignore. You're returning the address of a local variable. That address will become invalid when you leave classify()'s stack frame, before the caller has a chance to use it.

You only need to return the value of that variable instead:

return chars;