Re: [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
Ralf A. Quint wrote: At 11:52 AM 10/23/2011, Frank Peelo wrote: If I recall corerctly, that Modula-2 value for nil was the address the 8086 jumped to when released from RESET. So jumping through a pointer with value nil would be something you would notice pretty much immediately. Certainly not. Modula-2 was/is not depending on any CPU specifics, certainly not on anything x86, considering that the first computers Modula-2 ran on were the Xerox Alto and the Lilith, using M-Code, and both not even a remote connection to anything x86. And a lot of non-Wirth Modula-2 implementation ran on Motorola 68k before I can remember to have seen the first x86 implementation (not sure if it was FST or XDS)... We're getting off-topic. I *DID* specify that it was one, particular compiler that had that behaviour: if I recall correctly it was the first version that Logitech targeted at OS/2 (the one that bombed when you said BYTE(0)). I've also seen : used by a disassembler (Sourcer), possibly for the same value that Frank suggests. -- Mark Morgan Lloyd markMLl .AT. telemetry.co .DOT. uk [Opinions above are the author's, not those of his employers or colleagues] ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
On 22/10/11 10:01, Mark Morgan Lloyd wrote: Felipe Monteiro de Carvalho wrote: Free is how you release the memory allocated for a object. Free calls Destroy. Never call Destroy manually. When you implement the destructor you always implement Destroy, never change Free. A number of years ago, Matthew Jones's wife looked over his shoulder and caught him using Create and Destroy. Bit violent, isn't it? When he told the story somebody else pointed out that he should be using Create and Free, I find this anecdote makes it easy to remember. Nil is not a routine, it is a value, it means that the object is empty, it does not exist / is not allocated. Nil in existing implementations that I know is represented by the value zero. I've once seen a compiler using a non-zero value, I think it was the first Logitech protected-mode Modula-2 and it used :. Apart from that I think that C specifies that nil has a bit pattern of all zeros, and anything with even the slightest interest in compatibility sticks to this as a convention. Almost -- In C, NULL doesn't have to be all zeroes internally, but the value 0 in the source code represents the NULL pointer. The compiler is allowed to convert (void*)0 to a different bit pattern if the hardware requires it, but you'd need a pretty good reason to make life that complicated. So it would be a rare compiler that did that. If I recall corerctly, that Modula-2 value for nil was the address the 8086 jumped to when released from RESET. So jumping through a pointer with value nil would be something you would notice pretty much immediately. FP ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
At 11:52 AM 10/23/2011, Frank Peelo wrote: If I recall corerctly, that Modula-2 value for nil was the address the 8086 jumped to when released from RESET. So jumping through a pointer with value nil would be something you would notice pretty much immediately. Certainly not. Modula-2 was/is not depending on any CPU specifics, certainly not on anything x86, considering that the first computers Modula-2 ran on were the Xerox Alto and the Lilith, using M-Code, and both not even a remote connection to anything x86. And a lot of non-Wirth Modula-2 implementation ran on Motorola 68k before I can remember to have seen the first x86 implementation (not sure if it was FST or XDS)... Ralf ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
I understand Assigned as being the same as nil, so Assigned(Object) = Object nil I vaguely remember that it could be safer in some corner case, but I don't remember ever meting that. Free is how you release the memory allocated for a object. Free calls Destroy. Never call Destroy manually. When you implement the destructor you always implement Destroy, never change Free. Nil is not a routine, it is a value, it means that the object is empty, it does not exist / is not allocated. Nil in existing implementations that I know is represented by the value zero. The typical life-cycle of a object is: MyObject := TMyObject.Create; try MyObject.DoSomething(); finally MyObject.Free; end; To implement this object you should implement Create, DoSomething and Destroy. -- Felipe Monteiro de Carvalho ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
22.10.2011 10:20, Felipe Monteiro de Carvalho пишет: I understand Assigned as being the same as nil, so Assigned(Object) = Object nil I vaguely remember that it could be safer in some corner case, but I don't remember ever meting that. Method pointers? ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
On 22 October 2011 07:20, Felipe Monteiro de Carvalho
felipemonteiro.carva...@gmail.com wrote:
I understand Assigned as being the same as nil, so Assigned(Object)
= Object nil
I vaguely remember that it could be safer in some corner case, but I
don't remember ever meting that.
Free is how you release the memory allocated for a object. Free calls
Destroy. Never call Destroy manually. When you implement the
destructor you always implement Destroy, never change Free.
Nil is not a routine, it is a value, it means that the object is
empty, it does not exist / is not allocated. Nil in existing
implementations that I know is represented by the value zero.
The typical life-cycle of a object is:
MyObject := TMyObject.Create;
try
MyObject.DoSomething();
finally
MyObject.Free;
end;
To implement this object you should implement Create, DoSomething and
Destroy.
--
Felipe Monteiro de Carvalho
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This code is the source of my woes. SCStrings and FBreakStrings are part of
an object. They are repeatedly used in a loop and I need to free the memory
after the loop runs or free the space taken up by their strings
type
TRuntimeMonitor = class(TThread)
private
Frequency: Integer;
IniFile: TMemIniFile;
SCStrings: TStringList;
FBreakStrings: TStringList;
procedure DispatchOutput;
procedure DisplayRawOutput;
protected
procedure Execute; override;
public
constructor Create(CreateSuspended: Boolean);
end;
* * procedure InitVars;
begin
if not Assigned(SCStrings) then
SCStrings := TStringList.Create;
if not Assigned(FBreakStrings) then
FBreakStrings := TStringList.Create;
IniFile := TMemIniFile.Create('zxtyu');
end;
procedure FreeVars;
begin
IniFile.Free;
if Assigned(SCStrings) then
SCStrings.Free;
if Assigned(FBreakStrings) then
FBreakStrings.Free;
if Assigned(FBreakStrings) then
debugln('FBreakStrings is still assigned');
end;
*
*InitVars and FreeVars run in the Execute procedure of the thread*
*When the loop runs again Assigned in InitVars is false so as soon as those
FBreakStrings and SCStrings are accessed within the loop a SIGSEGV occurs.
So what I want to know is whether Assigned remains true when Free is
executed.
In the mean time instead of using Free in FreeVars I will set the text
property to the blank string to release the memory used by the strings.
--
Frank Church
===
http://devblog.brahmancreations.com
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RE : [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
Nil is not a routine, it is a value, it means that the object is empty, it does not exist / is not allocated. Nil in existing implementations that I know is represented by the value zero. The typical life-cycle of a object is: MyObject := TMyObject.Create; try MyObject.DoSomething(); finally MyObject.Free; end; One pitfall: the variable MyObject before MyObject := TMyObject.Create; is undefined and not necessarily nil. Variables are not initialized by default and can contain anything. In general, MyObject.Free does not set MyObject to nil neither. Good practice is to initialize pointer variables to nil and resetting them to nil after freeing them whenever assigned() or nil is going to be used. Ludo ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
RE : [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
When the loop runs again Assigned in InitVars is false so as soon as those FBreakStrings and SCStrings are accessed within the loop a SIGSEGV occurs. So what I want to know is whether Assigned remains true when Free is executed. See my previous answer. There is a helper function procedure FreeAndNil(var obj); that calls obj.free and sets obj to nil. So instead of SCStrings.Free; SCStrings:=nil; call FreeAndNil(SCStrings); Ludo ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: RE : [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
On 22 October 2011 08:14, Ludo Brands ludo.bra...@free.fr wrote: Nil is not a routine, it is a value, it means that the object is empty, it does not exist / is not allocated. Nil in existing implementations that I know is represented by the value zero. The typical life-cycle of a object is: MyObject := TMyObject.Create; try MyObject.DoSomething(); finally MyObject.Free; end; One pitfall: the variable MyObject before MyObject := TMyObject.Create; is undefined and not necessarily nil. Variables are not initialized by default and can contain anything. In general, MyObject.Free does not set MyObject to nil neither. Good practice is to initialize pointer variables to nil and resetting them to nil after freeing them whenever assigned() or nil is going to be used. Does that mean that Free itself reclaims the memory used by the object's fields and properties but does not release the memory used by the TObject or pointer itself, where as setting it to nil or executing Destroy does, or does Destroy do something different? Ludo ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal -- Frank Church === http://devblog.brahmancreations.com ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
Hello,
Instead of .Free in your FreeVars routine, you should use FreeAndNil(
theobject).
The correct lifecycle for class objects is :
var
anObject : TMyObject;
begin
anObject := nil; //not needed for class attributes. Required for local
function or global unit variables.
anObject := TMyObject.Create( params) //if an exception is raised here, the
anObject reference will still be nill.
try
//... do some code on anObject...
finally
FreeAndNil( anObject); //after this, the memory referenced by anObject is
freed, and the reference anObject is set to nil.
end;
end;
Hope this helps,
Thierry
- Mail Original -
De: Frank Church vfcli...@gmail.com
À: FPC-Pascal users discussions fpc-pascal@lists.freepascal.org
Envoyé: Samedi 22 Octobre 2011 09h08:17 GMT +01:00 Amsterdam / Berlin / Berne /
Rome / Stockholm / Vienne
Objet: Re: [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
On 22 October 2011 07:20, Felipe Monteiro de Carvalho
felipemonteiro.carva...@gmail.com wrote:
I understand Assigned as being the same as nil, so Assigned(Object)
= Object nil
I vaguely remember that it could be safer in some corner case, but I
don't remember ever meting that.
Free is how you release the memory allocated for a object. Free calls
Destroy. Never call Destroy manually. When you implement the
destructor you always implement Destroy, never change Free.
Nil is not a routine, it is a value, it means that the object is
empty, it does not exist / is not allocated. Nil in existing
implementations that I know is represented by the value zero.
The typical life-cycle of a object is:
MyObject := TMyObject.Create;
try
MyObject.DoSomething();
finally
MyObject.Free;
end;
To implement this object you should implement Create, DoSomething and Destroy.
--
Felipe Monteiro de Carvalho
___
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This code is the source of my woes. SCStrings and FBreakStrings are part of an
object. They are repeatedly used in a loop and I need to free the memory after
the loop runs or free the space taken up by their strings
type
TRuntimeMonitor = class(TThread)
private
Frequency: Integer;
IniFile: TMemIniFile;
SCStrings: TStringList;
FBreakStrings: TStringList;
procedure DispatchOutput;
procedure DisplayRawOutput;
protected
procedure Execute; override;
public
constructor Create(CreateSuspended: Boolean);
end;
procedure InitVars;
begin
if not Assigned(SCStrings) then
SCStrings := TStringList.Create;
if not Assigned(FBreakStrings) then
FBreakStrings := TStringList.Create;
IniFile := TMemIniFile.Create('zxtyu');
end;
procedure FreeVars;
begin
IniFile.Free;
if Assigned(SCStrings) then
SCStrings.Free;
if Assigned(FBreakStrings) then
FBreakStrings.Free;
if Assigned(FBreakStrings) then
debugln('FBreakStrings is still assigned');
end;
InitVars and FreeVars run in the Execute procedure of the thread
When the loop runs again Assigned in InitVars is false so as soon as those
FBreakStrings and SCStrings are accessed within the loop a SIGSEGV occurs. So
what I want to know is whether Assigned remains true when Free is executed.
In the mean time instead of using Free in FreeVars I will set the text property
to the blank string to release the memory used by the strings.
--
Frank Church
===
http://devblog.brahmancreations.com
___
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RE : RE : [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
Does that mean that Free itself reclaims the memory used by the object's fields and properties but does not release the memory used by the TObject or pointer itself, where as setting it to nil or executing Destroy does, or does Destroy do something different? All memory is released including TObject but the variable pointing to the TObject (fe: SCStrings) isn't set to nil. It still points to where the TObject was. Warning: setting a TObject variable to nil does not free the object unless it is reference counted! Strings and COM style interfaces are reference counted. Pascal is not VB. Ludo ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: RE : [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
On Sat, Oct 22, 2011 at 9:36 AM, Frank Church vfcli...@gmail.com wrote: Does that mean that Free itself reclaims the memory used by the object's fields and properties but does not release the memory used by the TObject or pointer itself, where as setting it to nil or executing Destroy does, or does Destroy do something different? No, you got parts of it wrong: *Free calls Destroy which releases the memory of the object itself. *Nothing releases the memory of the pointer to the object. You don't want to release the 4 or 8 bytes of the pointer. If you did release it, then your application would crash when trying to read the pointer to check if it is nil. *Destroy implements releasing the memory of the object. Free calls Destroy. *Setting the object to nil does just that. It changes the pointer to the object to have the value nil (zero). It does not release the pointer. -- Felipe Monteiro de Carvalho ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
Felipe Monteiro de Carvalho wrote: Free is how you release the memory allocated for a object. Free calls Destroy. Never call Destroy manually. When you implement the destructor you always implement Destroy, never change Free. A number of years ago, Matthew Jones's wife looked over his shoulder and caught him using Create and Destroy. Bit violent, isn't it? When he told the story somebody else pointed out that he should be using Create and Free, I find this anecdote makes it easy to remember. Nil is not a routine, it is a value, it means that the object is empty, it does not exist / is not allocated. Nil in existing implementations that I know is represented by the value zero. I've once seen a compiler using a non-zero value, I think it was the first Logitech protected-mode Modula-2 and it used :. Apart from that I think that C specifies that nil has a bit pattern of all zeros, and anything with even the slightest interest in compatibility sticks to this as a convention. -- Mark Morgan Lloyd markMLl .AT. telemetry.co .DOT. uk [Opinions above are the author's, not those of his employers or colleagues] ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: [fpc-pascal] How are Assigned, Free, Nil and Destroy related?
In our previous episode, Felipe Monteiro de Carvalho said: Nil is not a routine, it is a value, it means that the object is empty, it does not exist / is not allocated. Nil in existing implementations that I know is represented by the value zero. Look better in, euh, Free Pascal, and see what is assigned in this case:-) type t = procedure (a,b:integer) of object; var x : t; begin x:=nil; end. nil is not always zero. Sometimes it is double zero :-) ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
