[next] [tail] [up]

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:

_________________________________________________________________________________________________________
Properties

-- --- --- - ------------------- property-definition -class--|property identifier - --| ----property- specifiers- hint- directive-----------------property- interface-------

-- ----------------------- - -- property-interface -property- parameter- list-| : type- identifier ------------------------------------------------------------------ -index -integer- expression-|

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

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

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

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

--implements- specifier-implements -|-fully- qualified-identifier---------------- ---------,----------

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

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

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

--default- array- property-specifier ;-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 for regular properties, they can only be used for class properties.

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: 

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

Var
  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).

[next] [front] [up]