BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Pointer to AnsiString Goto page 1, 2, 3, 4  Next          BorlandTalk.com Forum Index -> C++ Builder (VCL Components Usage) View previous topic :: View next topic   Author Message Phil V Guest Posted: Thu Dec 04, 2003 12:24 pm    Post subject: Pointer to AnsiString Sorry guys, have a 'slow' moment here! I want to be able to have a pointer that points straight at the ansistring of the caption of a label. How do I do that? I tried, AnsiString* TheCaption = Label1->Caption; But of course this tries to assign the content of the string rather than the address of the string. I think I might need an '&' in there somewhere, but not sure where! As a second point; If i wanted to have a list of parameters in memory, pointed to directly like this so as to not need to worry about accessing the actual components how would I do it, and is it even possible? Thanks, Phil V -- "If it is called software, why is it so hard?" - Unknown Back to top Rodolfo Frino Guest Posted: Thu Dec 04, 2003 2:27 pm    Post subject: Re: Pointer to AnsiString I am not sure if this is what you want: Label1->Caption = "Hello Software World, 101010101010101010101010101010---"; String S = Label1->Caption; String* p = &S; ShowMessage(*p); if so, why not using? String S = Label1->Caption; ShowMessage(S); Rodolfo "Phil V" <pvalentine_REMOVE_THIS_ (AT) _REMOVE_THIS_vislink (DOT) com> wrote Quote: Sorry guys, have a 'slow' moment here! I want to be able to have a pointer that points straight at the ansistring of the caption of a label. How do I do that? I tried, AnsiString* TheCaption = Label1->Caption; But of course this tries to assign the content of the string rather than the address of the string. I think I might need an '&' in there somewhere, but not sure where! As a second point; If i wanted to have a list of parameters in memory, pointed to directly like this so as to not need to worry about accessing the actual components how would I do it, and is it even possible? Thanks, Phil V -- "If it is called software, why is it so hard?" - Unknown Back to top Phil V Guest Posted: Thu Dec 04, 2003 2:37 pm    Post subject: Re: Pointer to AnsiString Ok, I can see what you are doing here, but from the looks of things what you have done is copied the string from Label1->Caption into S, and then provided a pointer 'p' to S. What I was trying to achieve was to be able to have a list of pointers, to enable an easy update of the value in the components. For example: String P1* = Label1->Caption; String P2* = Memo1->Text; Then allowing for a simply function knowing nothing of the actual components to do something like: AnsiString MyTxt = "This is my text" P1 = MyTxt; // Make the CAPTION property contain "This is my text" P2 = MyTxt; // Make the TEXT property contain "This is my text" See, I don't actually want to copy the data, I want to be able to have a pointer directly to the property. How do i do this? Thanks, Phil "Rodolfo Frino" <MenInBlack (AT) nowhere (DOT) com> wrote Quote: I am not sure if this is what you want: Label1->Caption = "Hello Software World, 101010101010101010101010101010---"; String S = Label1->Caption; String* p = &S; ShowMessage(*p); if so, why not using? String S = Label1->Caption; ShowMessage(S); Rodolfo "Phil V" <pvalentine_REMOVE_THIS_ (AT) _REMOVE_THIS_vislink (DOT) com> wrote in message news:3fcf279d$1 (AT) newsgroups (DOT) borland.com... Sorry guys, have a 'slow' moment here! I want to be able to have a pointer that points straight at the ansistring of the caption of a label. How do I do that? I tried, AnsiString* TheCaption = Label1->Caption; But of course this tries to assign the content of the string rather than the address of the string. I think I might need an '&' in there somewhere, but not sure where! As a second point; If i wanted to have a list of parameters in memory, pointed to directly like this so as to not need to worry about accessing the actual components how would I do it, and is it even possible? Thanks, Phil V -- "If it is called software, why is it so hard?" - Unknown Back to top Bruce Salzman Guest Posted: Thu Dec 04, 2003 3:54 pm    Post subject: Re: Pointer to AnsiString AnsiString* TheCaption = &(Label1->Caption); AnsiString MyTxt = "This is my text"; *TheCaption = MyTxt; Regards, Bruce Back to top Chris Uzdavinis (TeamB) Guest Posted: Thu Dec 04, 2003 4:01 pm    Post subject: Re: Pointer to AnsiString " Bruce Salzman" <bruce (AT) nospam (DOT) org> writes: Quote: AnsiString* TheCaption = &(Label1->Caption); This is disastrous, since you're taking the address of a temporary, and it's immediately destroyed after the assignment finishes. The result is a pointer to an invalid region of memory that used to hold the temporary. Quote: AnsiString MyTxt = "This is my text"; *TheCaption = MyTxt; This is undefined, since TheCaption points to invalid memory and now you're dereferencing it. -- Chris (TeamB); Back to top Bruce Salzman Guest Posted: Thu Dec 04, 2003 4:04 pm    Post subject: Re: Pointer to AnsiString "Chris Uzdavinis (TeamB)" <chris (AT) uzdavinis (DOT) com> wrote Quote: " Bruce Salzman" <bruce (AT) nospam (DOT) org> writes: AnsiString* TheCaption = &(Label1->Caption); This is disastrous, since you're taking the address of a temporary, and it's immediately destroyed after the assignment finishes. The result is a pointer to an invalid region of memory that used to hold the temporary. I thought that only had to do with the pointer returned by c_str()... Why can't you take the address of the AnsiString itself? Back to top Phil V Guest Posted: Thu Dec 04, 2003 4:04 pm    Post subject: Re: Pointer to AnsiString Chris, I can see that in the 'second' section the mistake is dereferencing it, but what do you mean in the first section where you say it is a temporary? Does this NOT give me the memory location of the text that is making up the caption of the label then? Phil "Chris Uzdavinis (TeamB)" <chris (AT) uzdavinis (DOT) com> wrote Quote: " Bruce Salzman" <bruce (AT) nospam (DOT) org> writes: AnsiString* TheCaption = &(Label1->Caption); This is disastrous, since you're taking the address of a temporary, and it's immediately destroyed after the assignment finishes. The result is a pointer to an invalid region of memory that used to hold the temporary. AnsiString MyTxt = "This is my text"; *TheCaption = MyTxt; This is undefined, since TheCaption points to invalid memory and now you're dereferencing it. -- Chris (TeamB); Back to top Liz Albin Guest Posted: Thu Dec 04, 2003 4:14 pm    Post subject: Re: Pointer to AnsiString On Thu, 4 Dec 2003 14:37:31 -0000, Phil V wrote: Quote: See, I don't actually want to copy the data, I want to be able to have a pointer directly to the property. How do i do this? As the property is a temporary, I don't think it's a wise idea. -- liz Back to top Rodolfo Frino Guest Posted: Thu Dec 04, 2003 5:03 pm    Post subject: Re: Pointer to AnsiString Perhaps this is what you want void __fastcall TForm1::UpdatePropertyValues(String* p) { Label1->Caption = p[0]; Edit1->Text = p[1]; Button2->Caption = p[2]; } void __fastcall TForm1::Button1Click(TObject *Sender) { String* p = new String[1000]; p[0] = "TLABEL"; p[1] = "TEDIT"; p[2] = "TBUTTON"; UpdatePropertyValues(p); p[0] = "New Label caption"; p[1] = "New Edit1 text"; p[2] = "New Button2 caption"; UpdatePropertyValues(p); } Rodolfo "Phil V" <pvalentine_REMOVE_THIS_ (AT) _REMOVE_THIS_vislink (DOT) com> wrote Quote: Ok, I can see what you are doing here, but from the looks of things what you have done is copied the string from Label1->Caption into S, and then provided a pointer 'p' to S. What I was trying to achieve was to be able to have a list of pointers, to enable an easy update of the value in the components. For example: String P1* = Label1->Caption; String P2* = Memo1->Text; Then allowing for a simply function knowing nothing of the actual components to do something like: AnsiString MyTxt = "This is my text" P1 = MyTxt; // Make the CAPTION property contain "This is my text" P2 = MyTxt; // Make the TEXT property contain "This is my text" See, I don't actually want to copy the data, I want to be able to have a pointer directly to the property. How do i do this? Thanks, Phil "Rodolfo Frino" <MenInBlack (AT) nowhere (DOT) com> wrote in message news:3fcf4454 (AT) newsgroups (DOT) borland.com... I am not sure if this is what you want: Label1->Caption = "Hello Software World, 101010101010101010101010101010---"; String S = Label1->Caption; String* p = &S; ShowMessage(*p); if so, why not using? String S = Label1->Caption; ShowMessage(S); Rodolfo "Phil V" <pvalentine_REMOVE_THIS_ (AT) _REMOVE_THIS_vislink (DOT) com> wrote in message news:3fcf279d$1 (AT) newsgroups (DOT) borland.com... Sorry guys, have a 'slow' moment here! I want to be able to have a pointer that points straight at the ansistring of the caption of a label. How do I do that? I tried, AnsiString* TheCaption = Label1->Caption; But of course this tries to assign the content of the string rather than the address of the string. I think I might need an '&' in there somewhere, but not sure where! As a second point; If i wanted to have a list of parameters in memory, pointed to directly like this so as to not need to worry about accessing the actual components how would I do it, and is it even possible? Thanks, Phil V -- "If it is called software, why is it so hard?" - Unknown Back to top Bruce Salzman Guest Posted: Thu Dec 04, 2003 5:10 pm    Post subject: Re: Pointer to AnsiString "Phil V" <pvalentine_REMOVE_THIS_ (AT) _REMOVE_THIS_vislink (DOT) com> wrote Quote: Chris, I can see that in the 'second' section the mistake is dereferencing it, but what do you mean in the first section where you say it is a temporary? Does this NOT give me the memory location of the text that is making up the caption of the label then? Right...the properties of a TForm ARE temporaries! This pointer assignment/dereferencing would be OK for an AnsiString that stays in scope. The TForm property objects only exist long enough to bridge the gap between C++ and the Delphi code under the hood. It's easy to forget those tricks that VCL needs to mimic C++ classes. But I guess the moral is not to forget that they are not C++ classes! I miss OWL. :^( Regards, Bruce Back to top Pete Fraser Guest Posted: Thu Dec 04, 2003 5:34 pm    Post subject: Re: Pointer to AnsiString Probably because it's not a variable it's a 'property' which has a hidden getter and setter. The getter returns a temporary AnsiString which is what you have a pointer to so is of no use whatsoever. HTH Pete " Bruce Salzman" <bruce (AT) nospam (DOT) org> wrote Quote: "Chris Uzdavinis (TeamB)" <chris (AT) uzdavinis (DOT) com> wrote in message news:j5ad68incm.fsf (AT) explicit (DOT) atdesk.com... " Bruce Salzman" <bruce (AT) nospam (DOT) org> writes: AnsiString* TheCaption = &(Label1->Caption); This is disastrous, since you're taking the address of a temporary, and it's immediately destroyed after the assignment finishes. The result is a pointer to an invalid region of memory that used to hold the temporary. I thought that only had to do with the pointer returned by c_str()... Why can't you take the address of the AnsiString itself? Back to top Chris Uzdavinis (TeamB) Guest Posted: Thu Dec 04, 2003 5:39 pm    Post subject: Re: Pointer to AnsiString "Phil V" <pvalentine_REMOVE_THIS_ (AT) _REMOVE_THIS_vislink (DOT) com> writes: Quote: Chris, I can see that in the 'second' section the mistake is dereferencing it, but what do you mean in the first section where you say it is a temporary? Does this NOT give me the memory location of the text that is making up the caption of the label then? Asking for the Caption from a label returns a temporary AnsiString, I think. Since AnsiStrings are reference counted, and the string's not copied "by value" the reference count isn't increased, so when the return value is destroyed, the refcount goes to 0 and the string is deleted. Taking its address doesn't affect the refcount. Since AnsiStrings are very lightweight anyway, copying them by value is rather cheap, and avoids all sorts of problems. (The data isn't copied, since it's refcounted. Copying an AnsiString basically involves only copying a pointer, and increasing the refcount.) -- Chris (TeamB); Back to top Remy Lebeau (TeamB) Guest Posted: Thu Dec 04, 2003 6:55 pm    Post subject: Re: Pointer to AnsiString "Phil V" <pvalentine_REMOVE_THIS_ (AT) _REMOVE_THIS_vislink (DOT) com> wrote Quote: I want to be able to have a pointer that points straight at the ansistring of the caption of a label. Can't be done, sorry. The Caption is not an actual AnsiString instance to begin with, so you can't assign a pointer to it. Instead, the Caption is a pair of getter and setter methods that work with temporary AnsiString instances and retreive/update the caption text internally. Gambit Back to top Remy Lebeau (TeamB) Guest Posted: Thu Dec 04, 2003 6:56 pm    Post subject: Re: Pointer to AnsiString " Bruce Salzman" <bruce (AT) nospam (DOT) org> wrote Quote: AnsiString* TheCaption = &(Label1->Caption); That won't work. You are taking the address of a *temporary* value, not the property directly. Quote: *TheCaption = MyTxt; You are then updating the *temporary* value, not the property directly. Gambit Back to top Remy Lebeau (TeamB) Guest Posted: Thu Dec 04, 2003 7:02 pm    Post subject: Re: Pointer to AnsiString " Bruce Salzman" <bruce (AT) nospam (DOT) org> wrote Quote: I thought that only had to do with the pointer returned by c_str()... That is a different issue than the one being discussed here. The c_str() method returns a pointer to the AnsiString's internal memory buffer. The returned pointer is only valid as long as the AnsiString is not modified. As soon as the AnsiString is modified, its internal buffer is (usually) reallocated, and the pointer you obtained is no longer pointing to the same memory as before. This all assumes, however, that the AnsiString itself remains in memory the whole time you are using it. On the other hand... Quote: Why can't you take the address of the AnsiString itself? You can, but that is not the problem. As Chris already mentioned, the AnsiString returned by the Caption property is a *temporary* instance. Since you are not assigning it to a separate AnsiString variable, the temporary is going to go out of scope as soon as the statement is finished. When AnsiString goes out of scope, it is automatically freed, and you will be left with a pointer that is pointing to invalid memory since the AnsiString no longer exists. Anything you do to access that pointer will result in undefined behavior if you do not reassign it to valid memory first. 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