POINTER NULL CHECK

[Munair]

Consider the following code:

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dim handle as any ptr

if handle = NULL then ... ' OK
if not handle then ... ' error: wrong type argument to bit-complement

The error tells me that the NOT HANDLE test isn't supported. So is the first test HANDLE = NULL effective? Or is there more to it to see whether a pointer is assigned?

[jj2007]

This works fine for me:

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dim handle as any ptr

if handle = 0 then Print "NULL"
if not handle then Print "Handle="; handle

handle=1
if handle = 0 then Print "NULL"
if not handle then Print "Handle="; handle

Sleep

Output:

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NULL
Handle=0
Handle=1

Check also the output for

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if not handle then Print "not handle=";not handle

[fxm]

When compiling with gcc, complement a pointer is disallowed (works with gas).
gcc is stricter (or gas more lax)

Workaround:

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dim handle as any ptr

if handle = NULL then ... ' handle = 0
if handle then ... ' handle <> 0

or

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dim handle as any ptr

if handle = NULL then ... ' handle = 0
if handle <> NULL then ... ' handle <> 0

[MrSwiss]

So is the first test HANDLE = NULL effective?

No, it isn't, use: 0 (instead of NULL), since NULL might NOT be, defined!
(it isn't "native", aka: default defined, by compiler)

Btw: please, stop SHOUTING at us (all upper case, means that!)

[Munair]

So is the first test HANDLE = NULL effective?

No, it isn't, use: 0 (instead of NULL), since NULL might NOT be, defined!
(it isn't "native", aka: default defined, by compiler)

I defined it as 0 in my library. ;)

[Munair]

When compiling with gcc, complement a pointer is disallowed (works with gas).
gcc is stricter (or gas more lax)

Workaround:

Code: Select all

dim handle as any ptr

if handle = NULL then ... ' handle = 0
if handle then ... ' handle <> 0

or

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dim handle as any ptr

if handle = NULL then ... ' handle = 0
if handle <> NULL then ... ' handle <> 0

I noticed. Thanks.

[jj2007]

When compiling with gcc, complement a pointer is disallowed (works with gas).
gcc is stricter (or gas more lax)

The problem is maybe that some programmers don't pay much attention to the consequences:

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Dim handleI as integer
Dim handleB as boolean

handleI=123
handleB=123

if not handleI then
  print "not a handle: ";not handleI
else
  print "valid handle: "; handleI
endif

if not handleB then
  print "not a handle: ";not handleB
else
  print "valid handle: "; handleB
endif

Sleep

Output:

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not a handle: -124
valid handle: true

Or, in other words: integer <> boolean ;-)

[Munair]

That also includes POINTER <> BOOLEAN.

However, testing non-complementary boolean does work:

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dim handle as any ptr

if handle then ... ' works
if not handle then ... 'doesn't work

;)

[fxm]

When not testing Boolean but numeric values, the most correct expressions are for example (works with gas and gcc):

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dim handle as any ptr
dim k as integer

If ( handle = k ) Then ... ' OK
If ( handle <> k ) Then ... ' OK
If Not ( handle = k ) Then ... ' OK
If Not ( handle <> k ) Then ... ' OK


(including the case k = 0)

Another example, I don't like the syntax:

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If Len(string) Then ...


[Munair]

Another example, I don't like the syntax:

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If Len(string) Then ...


In QuickBASIC, testing with LEN produced faster code than with quotes:

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if len(MyString) then... ' faster
if MyString <> "" then ... 'slower

same for an empty string:

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if len(MyString) = 0 then... ' faster
if MyString = "" then ... ' slower

[fxm]

Another example, I don't like the syntax:

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If Len(string) Then ...


I thought rather:

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If Len(string) > 0 Then ...


(not omitting "> 0")

[dodicat]

not x seems to be simply -(x+1)
So a custom not seems trivial to make, handling integers/floats/pointers.

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#define pnot(x) -(cast(integer,x)+1)


print "integers"
for n as long=1 to 20
    var x=int(rnd*100-rnd*100)
print x,not x,pnot(x)
next
print

redim as long a(1 to 200001)

print "Pointers"
dim as any ptr p

for n as long=1 to 20
     p=@a(1+rnd*20000)
      if n=10 then p=0
    print p,pnot(p)
next

print
print "test 2000000 doubles"
for n as long=1 to 2000000
    var x=(rnd*1000000-rnd*1000000)
    if not(x)<>pnot(x) then print x,"error"
next
print "done"
sleep




 

[Munair]

I thought rather:

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If Len(string) > 0 Then ...

(not omitting "> 0")

I agree.

[jj2007]

Another example, I don't like the syntax:

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If Len(string) Then ...


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Dim MyString as string="123"

asm int 3
if len(MyString) Then
   print "ifLen"
endif
asm int 3
if len(MyString)>0 Then
   print "ifLenGtzero"
endif

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Address         Hex dump              Command                                 Comments
0040161B        ³.  CC                int3
0040161C        ³.  6A FF             push -1                                 ; ÚArg2 = -1
0040161E        ³.  8D45 E8           lea eax, [ebp-18]                       ; ³
00401621        ³.  50                push eax                                ; ³Arg1 => offset LOCAL.6
00401622        ³.  E8 49120000       call 00402870                           ; ÀTmpFile.00402870
00401627        ³.  85C0              test eax, eax
00401629        ³. 74 0E             jz short 00401639
0040162B        ³.  6A 01             push 1                                  ; ÚArg3 = 1
0040162D        ³.  68 40804000       push offset 00408040                    ; ³Arg2 = UNICODE "ifLen"
00401632        ³.  6A 00             push 0                                  ; ³Arg1 = 0
00401634        ³.  E8 C7080000       call 00401F00                           ; ÀTmpFile.00401F00
00401639        ³>  CC                int3
0040163A        ³.  6A FF             push -1                                 ; ÚArg2 = -1
0040163C        ³.  8D45 E8           lea eax, [ebp-18]                       ; ³
0040163F        ³.  50                push eax                                ; ³Arg1 => offset LOCAL.6
00401640        ³.  E8 2B120000       call 00402870                           ; ÀTmpFile.00402870
00401645        ³.  85C0              test eax, eax
00401647        ³. 7E 0E             jle short 00401657
00401649        ³.  6A 01             push 1                                  ; ÚArg3 = 1
0040164B        ³.  68 4C804000       push offset 0040804C                    ; ³Arg2 = UNICODE "ifLenGtzero"
00401650        ³.  6A 00             push 0                                  ; ³Arg1 = 0
00401652        ³.  E8 A9080000       call 00401F00                           ; ÀTmpFile.00401F00

The only difference: jz (jump zero) vs jle (jump less or equal). Since it is difficult to imagine a string with a negative length, the practical relevance is rather low...

[fxm]

Another example, I don't like the syntax:

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If Len(string) Then ...


Obviously this syntax compiles well and works but this code BASIC is not "clean" in my humble opinion, because the expression 'Len (string)' is neither true nor false, but conversely the expression 'Len (string) = 0' is true or false.
Here, I talk about the quality of the source code!