BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  TDataSet::operator[]          BorlandTalk.com Forum Index -> C++ Builder (VCL Components Usage) View previous topic :: View next topic   Author Message Tom Byars Guest Posted: Mon Apr 10, 2006 5:03 pm    Post subject: TDataSet::operator[] I only just realised this existed and thought it was a more compact way of typing Tbl->Values["fieldname"] however, if I type Tbl["fieldname"] I get an error stating Cannot convert 'char*' to 'int'. What am I missing? Back to top Vladimir Stefanovic Guest Posted: Mon Apr 10, 2006 5:03 pm    Post subject: Re: TDataSet::operator[] If it's a question of T(ADO)Table, then you can do something like this: // if field_name is string ADOTable1->FieldByName("field_name")->AsString or: // type will be determined internaly ADOTable1->FieldValues["field_name"] or: // 0 is the first field, and it's string ADOTable1->Fields->Fields[0]->AsString Probably there are some other notations, and ways, but these three came to my mind. -- Best Regards, Vladimir Stefanovic "Tom Byars" <tam118118 (AT) hotmail (DOT) com> wrote in message news:443a8581 (AT) newsgroups (DOT) borland.com... Quote: I only just realised this existed and thought it was a more compact way of typing Tbl->Values["fieldname"] however, if I type Tbl["fieldname"] I get an error stating Cannot convert 'char*' to 'int'. What am I missing? Back to top Remy Lebeau (TeamB) Guest Posted: Mon Apr 10, 2006 7:03 pm    Post subject: Re: TDataSet::operator[] "Tom Byars" <tam118118 (AT) hotmail (DOT) com> wrote in message news:443a8581 (AT) newsgroups (DOT) borland.com... Quote: I only just realised this existed and thought it was a more compact way of typing Tbl->Values["fieldname"] Which version of BCB are you using? The subscript '[]' operator is only implemented for VCL properties in BCB 6 and BDS 2006 that are marked as 'default' in the Delphi source code. Assuming 'Tbl' is a TTable, then there is no Values[] property in TTable. What exactly is 'Tbl' declared as? Quote: however, if I type Tbl["fieldname"] I get an error stating Cannot convert 'char*' to 'int'. What am I missing? Operators have to be called on an object *reference*, not an object *pointer*. Since all VCL objects are dynamically allocated on the heap, you have to deference the pointer in order to reach the actual object in order to call operators on it. This is documented in the help file: Subscript operators One Object Pascal feature that has not been available to C++Builder users is the concept of a default property (an indexed property that is implied if you use a subscript operator on the class). VCL and CLX classes that have default properties in Object Pascal now have a subscript operator in C++ to provide much of the same convenience. It does differ in that you must dereference the object pointer to use it. For example, now instead of writing Strings1->Strings[i] you can write (*Strings1)[i] So, assuming that 'Tbl' has a 'default' property Values[], then you can write: (*Tbl)["fieldname"] Gambit Back to top Remy Lebeau (TeamB) Guest Posted: Mon Apr 10, 2006 8:03 pm    Post subject: Re: TDataSet::operator[] "Remy Lebeau (TeamB)" <no.spam (AT) no (DOT) spam.com> wrote in message news:443aa838$1 (AT) newsgroups (DOT) borland.com... Quote: Assuming 'Tbl' is a TTable, then there is no Values[] property in TTable. What exactly is 'Tbl' declared as? Nevermind. I see that you were referring to TDataSet. TDataSet does not have a Values[] property. It does, however, have a FieldValues[] property instead. Gambit Back to top Tom Byars Guest Posted: Tue Apr 11, 2006 1:03 pm    Post subject: Re: TDataSet::operator[] Sorry Remy, I should of course have written Tbl->FieldValues["fieldname"] instead of Tbl->Values["fieldname"] (Tbl was meant to denote a TTable). (*Tbl)["fieldname"] does return Tbl->FieldValues["fieldname"] although the (*) kinda retracts from brevity. Thanks to yourself and Vladimir. Back to top Tom Byars Guest Posted: Tue Apr 11, 2006 1:03 pm    Post subject: Re: TDataSet::operator[] Any idea why this doesn't work Tbl->Edit(); (*Tbl)["fieldname"]=5; Tbl->Post(); compiles and executes OK but, the field value isn't changed to 5. Also, trying to inspect (*Tbl)["fieldname"] gives the error "E2094: 'operator +' not implemented in type 'Dbtables::TTable' for arguments of type 'char *'. "Tom Byars" <tam118118 (AT) hotmail (DOT) com> wrote in message news:443b9642 (AT) newsgroups (DOT) borland.com... Quote: Sorry Remy, I should of course have written Tbl->FieldValues["fieldname"] instead of Tbl->Values["fieldname"] (Tbl was meant to denote a TTable). (*Tbl)["fieldname"] does return Tbl->FieldValues["fieldname"] although the (*) kinda retracts from brevity. Thanks to yourself and Vladimir. Back to top Remy Lebeau (TeamB) Guest Posted: Tue Apr 11, 2006 7:03 pm    Post subject: Re: TDataSet::operator[] "Tom Byars" <tam118118 (AT) hotmail (DOT) com> wrote in message news:443b9fbb (AT) newsgroups (DOT) borland.com... Quote: Any idea why this doesn't work Does this work? Tbl->Edit(); Tbl->FieldValues["fieldname"] = 5; Tbl->Post(); If that works, then there is no reason for (*Tbl)["fieldname"] to not work. Gambit Back to top Tom Byars Guest Posted: Tue Apr 11, 2006 10:03 pm    Post subject: Re: TDataSet::operator[] Remy Tbl->Edit(); Tbl->FieldValues["fieldname"] = 99; (*Tbl)["fieldname"] = 5; Tbl->Post(); changes the field value to 99 ???? "Remy Lebeau (TeamB)" <no.spam (AT) no (DOT) spam.com> wrote in message news:443bf6b5$2 (AT) newsgroups (DOT) borland.com... Quote: "Tom Byars" <tam118118 (AT) hotmail (DOT) com> wrote in message news:443b9fbb (AT) newsgroups (DOT) borland.com... Any idea why this doesn't work Does this work? Tbl->Edit(); Tbl->FieldValues["fieldname"] = 5; Tbl->Post(); If that works, then there is no reason for (*Tbl)["fieldname"] to not work. Gambit Back to top Remy Lebeau (TeamB) Guest Posted: Tue Apr 11, 2006 11:03 pm    Post subject: Re: TDataSet::operator[] "Tom Byars" <tam118118 (AT) hotmail (DOT) com> wrote in message news:443c1ae4 (AT) newsgroups (DOT) borland.com... Quote: Tbl->Edit(); Tbl->FieldValues["fieldname"] = 99; (*Tbl)["fieldname"] = 5; Tbl->Post(); changes the field value to 99 ???? Are you asking me or telling me? ;-) If you look in db.hpp, you can exactly what the '[]' operator is really doing: public: Variant operator[](AnsiString FieldName) { return FieldValues[FieldName]; } Because the Variant is being returned by value instead of by reference, and the FieldValues property getter method is always being invoked, you will not be able to use the (*)[] syntax to assign values, only read them. Gambit Back to top Remy Lebeau (TeamB) Guest Posted: Wed Apr 12, 2006 12:03 am    Post subject: Re: TDataSet::operator[] "Remy Lebeau (TeamB)" <no.spam (AT) no (DOT) spam.com> wrote in message news:443c2ea4$1 (AT) newsgroups (DOT) borland.com... Quote: Because the Variant is being returned by value instead of by reference, and the FieldValues property getter method is always being invoked, you will not be able to use the (*)[] syntax to assign values, only read them. I have reported the issue to QC #27241, as the issue still exists in BDS and effects every class that has a default property. Gambit Back to top Tom Byars Guest Posted: Wed Apr 12, 2006 9:03 am    Post subject: Re: TDataSet::operator[] OK Remy, thanks. "Remy Lebeau (TeamB)" <no.spam (AT) no (DOT) spam.com> wrote in message news:443c2ea4$1 (AT) newsgroups (DOT) borland.com... Quote: "Tom Byars" <tam118118 (AT) hotmail (DOT) com> wrote in message news:443c1ae4 (AT) newsgroups (DOT) borland.com... Tbl->Edit(); Tbl->FieldValues["fieldname"] = 99; (*Tbl)["fieldname"] = 5; Tbl->Post(); changes the field value to 99 ???? Are you asking me or telling me? ;-) If you look in db.hpp, you can exactly what the '[]' operator is really doing: public: Variant operator[](AnsiString FieldName) { return FieldValues[FieldName]; } Because the Variant is being returned by value instead of by reference, and the FieldValues property getter method is always being invoked, you will not be able to use the (*)[] syntax to assign values, only read them. 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