How to translate this piece of code to FB?

[hungnguyengia]

https://github.com/QuantumLeaps/OOP-in- ... on/shape.h

It's a const pointer to a struct. With declaration like this:

declare sub Shape_ctor(byval me as const Shape ptr, byval x as long, byval y as long)

Passing as a pointer byval is real troublesome. It's impossible to just translate as is from the C code:

Dim as Shape s1, s2

Shape_ctor(@s1, 0, 1)

The compiler will not stop complaining about Array not dimensioned.

Changing this to pass byref solved part of the problems. But now we encountered const:

declare sub Shape_ctor(byref me as const Shape, byval x as long, byval y as long)

In shape.bas (translated from shape.c) if you use me.x = x the compiler will complain because me is const. The only way is to remove const:

declare sub Shape_ctor(byref me as Shape, byval x as long, byval y as long)

But this will lose the protection of const.

I know FB supports OOP. I just wanted to translate the C code. Thanks.

[grindstone]

To protect the pointer the Const has to be put before the Ptr:

Code: Select all

Type shape
   x As Short '/* x-coordinate of Shape's position */
   y As Short ' /* y-coordinate of Shape's position */
End Type

Declare Sub Shape_ctor(ByVal me As Shape Const Ptr, ByVal x As Short, ByVal y As Short)

Dim As Shape s1, s2

Shape_ctor(@s1, 0, 1)

Sub Shape_ctor(ByVal me As Shape Const Ptr, ByVal x As Short, ByVal y As Short)
	Print me
	'me = 0 'throws compiler error if uncommented
	Print me
	Print x
	Print y
End Sub

Sleep

while

Code: Select all

int16_t Shape_getX(Shape const * const me);

translates to

Code: Select all

Declare Function Shape_getX(me As Const Shape Const Ptr) As Short

and protects both the content of the struct and the pointer.

BTW: int16_t is Short, not Long

[hungnguyengia]

What is the point of this, my friend? Shape_ctor is used to emulate a constructor, you set it to print the value of the attributes into the screen so it's useless!

I already have working translation my friend. The only thing I lack is I have to strip the const otherwise it will not work.

shape.bi

Code: Select all

#pragma once

type Shape
  as integer x
  as integer y
end type

declare sub Shape_ctor(byref me as Shape, byval x as integer, byval y as integer)
declare sub Shape_moveby(byref me as Shape, byval dx as integer, byval dy as integer)
declare function Shape_getX(byref me as Shape) as integer
declare function Shape_getY(byref me as Shape) as integer

shape.bas

Code: Select all

#include once "shape.bi"

sub Shape_ctor(byref me as Shape, byval x as integer, byval y as integer)
  me.x = x
  me.y = y
end sub

sub Shape_moveby(byref me as Shape, byval dx as integer, byval dy as integer)
  me.x += dx
  me.y += dy
end sub

function Shape_getX(byref me as Shape) as integer
  return me.x
end function

function Shape_getY(byref me as Shape) as integer
  return me.y
end function

main.bas

Code: Select all

#include once "shape.bi"

#lang "fblite"

dim as Shape s1, s2

Shape_ctor(s1, 0, 1)
Shape_ctor(s2, -1, 2)

print "Shape s1(x = "; Shape_getX(s1); ", y = "; Shape_getY(s1); ")"
print "Shape s2(x = "; Shape_getX(s2); ", y = "; Shape_getY(s2); ")"

Shape_moveBy(s1, 2, -4)
Shape_moveBy(s2, 1, -2)

print "Shape s1(x = "; Shape_getX(s1); ", y = "; Shape_getY(s1); ")"
print "Shape s2(x = "; Shape_getX(s2); ", y = "; Shape_getY(s2); ")"

sleep

BTW, did you even try to compile your code to see if it could pass the compiler at all? I'm pretty sure I tried everything possible with passing by value a pointer but the compiler will not let me go any further and constantly flooding me with Array not dimensioned error.

[grindstone]

What is the point of this, my friend? Shape_ctor is used to emulate a constructor, you set it to print the value of the attributes into the screen so it's useless!

My intention was to show how the Const keyword works in FB. And because your shape.bi isn't available here, I wrote that litte example sub.

And yes, I compiled that snippet here, and it works.

[grindstone]

Alright, is it this what you wanted?

shape.bi

Code: Select all

#Pragma Once

Type Shape
  As Integer x
  As Integer y
End Type

Declare Sub Shape_ctor(me As Shape Const Ptr, ByVal x As Integer, ByVal y As Integer)
Declare Sub Shape_moveby(me As Shape Const Ptr, ByVal dx As Integer, ByVal dy As Integer)
Declare Function Shape_getX(me As Const Shape Const Ptr) As Integer
Declare Function Shape_getY(me As Const Shape Const Ptr) As Integer

shape.bas

Code: Select all

#Include Once "shape.bi"

Sub Shape_ctor(me As Shape Const Ptr, ByVal x As Integer, ByVal y As Integer)
  me->x = x
  me->y = y
End Sub

Sub Shape_moveby(me As Shape Const Ptr, ByVal dx As Integer, ByVal dy As Integer)
  me->x += dx
  me->y += dy
End Sub

Function Shape_getX(me As Const Shape Const Ptr) As Integer
  Return me->x
End Function

Function Shape_getY(me As Const Shape Const Ptr) As Integer
  Return me->y
End Function

main.bas

Code: Select all

#Include Once "shape.bas"

#Lang "fblite"

Dim As Shape s1, s2

Shape_ctor(@s1, 0, 1)
Shape_ctor(@s2, -1, 2)

Print "Shape s1(x = "; Shape_getX(@s1); ", y = "; Shape_getY(@s1); ")"
Print "Shape s2(x = "; Shape_getX(@s2); ", y = "; Shape_getY(@s2); ")"

Shape_moveby(@s1, 2, -4)
Shape_moveby(@s2, 1, -2)

Print "Shape s1(x = "; Shape_getX(@s1); ", y = "; Shape_getY(@s1); ")"
Print "Shape s2(x = "; Shape_getX(@s2); ", y = "; Shape_getY(@s2); ")"

Sleep

[dodicat]

In c/c++ passing a struct to a sub has an overload, and is considered bad practise, so the pointer to the struct is passed as const so the address is not altered.
Note that fb defaults to passing a udt byref, so the same problem doesn't arise.
In fb the default is byval for a pointer anyway, so it not altered.
Even passing the pointer byref is OK in this example because you don't alter me the pointer address, you just alter the pointer value.
So Grindstones code could be cleared of const.
But of course the failsafe way is to use const because you might inadvertently change the address if the sub was convoluted and big.

[hungnguyengia]

Yes, worked. Thank you, grindstone.