What are published class properties in Pascal?

In Pascal, a class declaration may include multiple private, protected, published, and public blocks as required. The visibility of these blocks differs depending on the selected access modifier.

Typically, the published access modifier is nearly similar to the public modifier and it has the following characteristics:

  • Denotes a section within a class that is accessible from any place.

  • The compiler generates RTTIRun Time Type Information information for the methods, fields, and properties in the published section. This information is needed for automatic streaming.

  • The published specifier can be used within classes only, not interfaces.

  • The fields defined in the published section must be of the ordinal type, real type, strings, or sets. They cannot be arrays.

  • A class can only contain a published section if compiled in $M+ mode.

Let's go over the following example to get a grasp on this subject:

Code example

This example shows how to declare a class containing a published section:

program hello;
{$mode objfpc}{$M+}
type   
  myClass = Class
  published 
    Function myFirstFunc  : Integer;  
  end;
  Function myClass.myFirstFunc : Integer;  
  begin  
    Result := 5;
  end; 
var  
  cl1 : myClass;
begin
  writeln('Start...');
  cl1:=myClass.create;
  writeln('cl1.myFirstFunc  = ',cl1.myFirstFunc());
  writeln('End...');
end.

Code explanation

Let's go over the code widget above:

  • Line 1: Refer to the program header. It is used for backward compatibility, and it is ignored by the compiler.

  • Line 2: Directives to the compiler to allow defining classes containing a published section.

  • Lines 4–7: Define a class called myClass using the Class keyword. Include a published section within this class containing a member function named myFirstFunc.

  • Lines 9–12: Define the previously declared myFirstFunc function. This function returns an integer value.

  • Lines 13–14: Declare a variable named cl1 with the myClass type.

  • Line 17: Create an instance of the myClass class and store it in the previously declared cl1 variable.

  • Line 18: Using this class instance, invoke the myFirstFunc function
    and print out its result.

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