How to initialise an empty pointer (like nullptr in C++)?

[parketh]

In languages like C++, it is possible to create a pointer that points to nothing (nullptr). This is useful for initialising objects that point to other variables - but where the pointer is not yet defined at the point of initialisation.

Is it possible to do this in Cairo, or if not, what is the recommended workaround?

[yuval-weiss]

I'm not 100% sure, but I think the closest thing to that is let nullptr = cast(0, felt*). It's a pointer to 0, same as an uninitialized memory cell. I guess it's more of an "undefined pointer" rather than a null pointer.

Also, often times when passing pointers around, you have an additional <POINTER_VAR_NAME>_size or <POINTER_VAR_NAME>_len param that is passed along with the pointer. In that case, the size/len param will be 0.

[parketh]

Thanks very much for your help with this! I initially closed the issue because it seems to work well for pointers to regular structs. However I would like to use this with a dictionary, and let nullptr = cast(0, DictAccess*) returns the following error:

Error: [code:49:5](javascript:current_page().move_cursor(49 - 1, 5 - 1)): Cannot compare 'felt' and 'starkware.cairo.common.dict_access.DictAccess*'.
    assert values[1] = cast(0, DictAccess*)

I thought maybe it is because I need to define nullptr as a pointer to DictAccess*, but that didnt work either and resulted in a similar error..