How to access a variable that a pointer pointed to? /flexbasic

pik33

There is an example in documentation:

  var x = new ubyte(10)  ' allocate 11 (not 10) bytes and return a pointer to it
  x(1) = 1               ' set a variable in it

How to

  var x = new ubyte  ' allocate 1 byte and return a pointer to it
  (????)= 1                     '   set a variable in it

?

evanh

Why not just use a single compile time declared variable? The usual reason to allocate from heap or stack is for large runtime filled arrays.

pik33

This question is the side effect of problems with passing and returning classes and multiple valuse to/from functions. I tried a workaround involving passing/returning a pointer to temporary allocated variable, then I realized that I don't know how to access a variable having a pointer in FlexBasic (in normal way, not with acrobations using poke and cast, or treating a variable as one-element array). In C you use * to do this, in Pascal there is ^, something should be here, too.

ersmith

Dereferencing a pointer in BASIC is always done with array notation, like p(0) (or p(1) if you have set 1 as the default array base).

However, for passing variables to subroutines and functions it's much easier to use byref parameters than pointers. That is, instead of:

function expr(ct as integer) as expr_result, integer
  ...
  return t1,ct
end function

a,ct = expr(ct)

do

function expr(byref ct as integer) as expr_result
...
a = expr(ct)  ' ct is updated to the new value by expr

The byref will cause ct to be passed "by reference", which means internally a pointer to the variable will be passed and changes to ct inside the function will affect the caller's variable.

evanh

Ah, right. This is why Chip made Spin without type checking. He much preferred assembly's freewheeling over C's myriad of referencing syntax (and presumably Pascal's too).

Mickster

Hope I'm not hijacking the thread but do we have code pointers in FlexBasic or have I missed it?

I realise that we have On-Gosub but is this a jump table?

Craig

ersmith

@Mickster said:
Hope I'm not hijacking the thread but do we have code pointers in FlexBasic or have I missed it?

We have pointers to functions/subroutines; is that what you meant?

I realise that we have On-Gosub but is this a jump table?

Yes, on-gosub and on-goto (and select/case) are implemented internally with a jump table, if appropriate (the compiler tries to guess whether a jump table or series of if statements would be more efficient).

Mickster

@ersmith

Oh the select/case is interesting...I'll test it

Craig