Run-time type information. Part 1

This article provides an introduction to run-time type information (RTTI). RTTI is information that Delphi stores in the compiled binary file of your application. RTTI is associated with published properties of a component. One of example of how RTTI impacts your everyday use of Delphi is Object Inspector, which displays the names of published properties and is able to edit them.

Documentation and compatibility.

RTTI is undocumented. The only place where you can find information about RTTI is typinfo.pas source file, which is located in X:\delphi\Source\VCL directory. Since typinfo.pas is undocumented, it may be changed at any way time in any new release of Delphi. As result, it's possible that changes in typinfo.pas will require you to make modifications to your programs in order to recompile them under the further release of Delphi if you use RTTI.

All functions and procedures as well as type declarations used by test application are the same under Delphi 3 and Delphi 4. I did not test it under Delphi 5 and any version of C++ Builder.

Getting RTTI for enumerated types.

Enumerated types are used throughout Delphi. An example - TBitBtnKind type, which declares valid values for the Kind property of TBitBtn component.

Type
TBitBtnKind = (bkCustom, bkOK, bkCancel, bkHelp, bkYes, bkNo, bkClose, bkAbort, bkRetry, bkIgnore, bkAll);

You can do two things with enumerated types using RTTI. You can retrieve a sting - name of the particular value within enum and identify ordinal value from the string representation of the value.

1. Obtain a string equivalent of enum value.
function GetEnumName(TypeInfo: PTypeInfo; Value: Integer): string;

2. Obtain ordinal value from the string representation
function GetEnumValue(TypeInfo: PTypeInfo; const Name: string): Integer;

The first argument of either function is a pointer to the RTTI for an enumerated type. You should use the built-in Typeinfo function to retrieve this information. The second argument is a string or an ordinal correspondingly.

Code Example

Var

S:string;
I:integer;

Begin

S:=GetEnumName(TypeInfo(TBitBtnKind),3);
ShowMessage(S); // displays "bkHelp"
I:=GetEnumValue(TypeInfo(TBitBtnKind),s);
ShowMessage(IntToStr(i)); // displays 3

End;

The first message box will display "bkHelp" string - a string for fourth value within TBitButtonKind type (enumerated types are zero based).

The second message box will display 3 - the ordinal for "bkHelp".

Test Project.

The use of these functions is demonstrated in the test project, which also demonstrates the use of the BitBtn component from the "Additional" page of the component palette. Figure 1 shows the main form of this project as it might appear while running. Listing 1 contains a source.

Figure 1.

unit Unit1;