6.7.1 Definition

Classes can contain properties as part of their fields list. A property acts like a normal field, i. e. its value can be retrieved or set, but it allows to redirect the access of the field through functions and procedures. They provide a means to associate an action with an assignment of or a reading from a class “field”. This allows e. g. checking that a value is valid when assigning, or, when reading, it allows to construct the value on the fly. Moreover, properties can be read-only or write only. The prototype declaration of a property is as follows:


 --              ---    ---       -        -------------------
   property definition  -class--|property  identifier  -             --|
----property specifiers hintdirective -----------------property interface-----

 --              ----------------------- -           --
   property interface  -property parameter list : type identifier
   -index -integerconstant--|

--                 -  ---                 --  ------------------
  property parameter list [  -parameter declaration-|]

 --property specifiers--|-------------|-------------------
                   -read specifier-- ----write specifier----|
----------------------------------implements specifier---------------
   -            --|-            -| -                     --|
    default specifier    stored specifier   defaultarraypropertyspecifier

--read specifier- read -field or method-------------------------------

--write specifier-write- field or method------------------------------

--implements specifier-implements -|-identifier -------------------------
                               ---, ----

--default specifier--default------------------------------------------
                |        -constant-| |
                ------nodefault ------|

--stored specifier-stored---constant----------------------------------
                      -identifier -|

--            ----           ------------------------------------
  field or method  - field identifier  -|
                 method identifier

--defaultarraypropertyspecifier- ;- default -------------------------------

A read specifier is either the name of a field that contains the property, or the name of a method function that has the same return type as the property type. In the case of a simple type, this function must not accept an argument. In case of an array property, the function must accept a single argument of the same type as the index. In case of an indexed property, it must accept a integer as an argument.

A read specifier is optional, making the property write-only. Note that class methods cannot be used as read specifiers.

A write specifier is optional: If there is no write specifier, the property is read-only. A write specifier is either the name of a field, or the name of a method procedure that accepts as a sole argument a variable of the same type as the property. In case of an array property, the procedure must accept two arguments: the first argument must have the same type as the index, the second argument must be of the same type as the property. Similarly, in case of an indexed property, the first parameter must be an integer.

The section (private, published) in which the specified function or procedure resides is irrelevant. Usually, however, this will be a protected or private method.

For example, given the following declaration:

  MyClass = Class  
    Field1 : Longint;  
    Field2 : Longint;  
    Field3 : Longint;  
    Procedure  Sety (value : Longint);  
    Function Gety : Longint;  
    Function Getz : Longint;  
    Property X : Longint Read Field1 write Field2;  
    Property Y : Longint Read GetY Write Sety;  
    Property Z : Longint Read GetZ;  
  MyClass : TMyClass;

The following are valid statements:

WriteLn ('X : ',MyClass.X);  
WriteLn ('Y : ',MyClass.Y);  
WriteLn ('Z : ',MyClass.Z);  
MyClass.X := 0;  
MyClass.Y := 0;

But the following would generate an error:

MyClass.Z := 0;

because Z is a read-only property.

What happens in the above statements is that when a value needs to be read, the compiler inserts a call to the various getNNN methods of the object, and the result of this call is used. When an assignment is made, the compiler passes the value that must be assigned as a parameter to the various setNNN methods.

Because of this mechanism, properties cannot be passed as var arguments to a function or procedure, since there is no known address of the property (at least, not always).