| losing pointers when concatenating array of structures [message #94050] |
Sun, 08 January 2017 22:35  |
MarioIncandenza
Messages: 231
Registered: February 2005
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Senior Member |
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Hi IDL Wizards,
That topic is a mouthful, but I haven't encountered a tricky problem like this in a while, and I wanted your help to understand what's really going on here. Here's the code sample:
for i=0,1 do begin
undefine,struct
struct=create_struct('array',ptr_new(lindgen(i+1)))
if n_elements(structarray) eq 0 then structarray=struct else structarray=[structarray,struct]
help,*structarray[i].array ; looking for error
; this is where the error will happen
if(i gt 0) then help,*structarray[i-1].array
endfor
end
Here's the result:
% Compiled module: $MAIN$.
<PtrHeapVar23543>
LONG = Array[1]
<PtrHeapVar23544>
LONG = Array[2]
% Invalid pointer: <POINTER (<PtrHeapVar23543>)>.
% Execution halted at: $MAIN$ 9
Here's the fix:
if n_elements(structarray) eq 0 then structarray=temporary(struct) else structarray=[structarray,temporary(struct)]
If you use TEMPORARY(), this works. If you do not, it fails. Can someone help me understand this?
Happy New Year's everyone,
--Edward H.
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| Re: losing pointers when concatenating array of structures [message #94053 is a reply to message #94052] |
Mon, 09 January 2017 11:05  |
Jim Pendleton
Messages: 165
Registered: November 2011
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Senior Member |
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On Monday, January 9, 2017 at 9:44:30 AM UTC-7, Edward Hyer wrote:
> On Monday, January 9, 2017 at 2:09:58 AM UTC-8, Heinz Stege wrote:
>> Well, pointers are tricky sometimes. I will try to explain.
>
> That is a good answer, thank you Heinz. So here's a question: what if I want to make a *copy* of a structure-containing-pointers, creating new heap variables for all the pointers in the structure? Maybe this cannot be done automatically? This is the problem that had me searching the IDL help for a STRUCT_COPY procedure.
A quick trick for making a *deep copy* is to SAVE the structure to an output file, then RESTORE the data, making sure you don't accidentally overwrite your original variable when you restore.
All included heap variable references (as well as all their dependencies) will be duplicated.
If you need a procedure that is not as deep, you will need to write your own logic. In ENVI, this is accomplished through dehydrate/rehydrate techniques.
Jim P.
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