Quick start manual

Classes and objects 7-23
Properties
Property overrides and redeclarations
A property declaration that doesn’t specify a type is called a property override.
Property overrides allow you to change a property’s inherited visibility or specifiers.
The simplest override consists only of the reserved word property followed by an
inherited property identifier; this form is used to change a property’s visibility. For
example, if an ancestor class declares a property as protected, a derived class can
redeclare it in a public or published section of the class. Property overrides can
include read, write, stored, default, and nodefault directives; any such directive
overrides the corresponding inherited directive. An override can replace an inherited
access specifier, add a missing specifier, or increase a property’s visibility, but it
cannot remove an access specifier or decrease a property’s visibility. An override can
include an implements directive, which adds to the list of implemented interfaces
without removing inherited ones.
The following declarations illustrate the use of property overrides.
type
TAncestor = class
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protected
property Size: Integer read FSize;
property Text: string read GetText write SetText;
property Color: TColor read FColor write SetColor stored False;
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end;
type
TDerived = class(TAncestor)
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protected
property Size write SetSize;
published
property Text;
property Color stored True default clBlue;
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end;
The override of Size adds a write specifier to allow the property to be modified. The
overrides of Text and Color change the visibility of the properties from protected to
published. The property override of Color also specifies that the property should be
filed if its value isn’t clBlue.
A redeclaration of a property that includes a type identifier hides the inherited
property rather than overriding it. This means that a new property is created with the
same name as the inherited one. Any property declaration that specifies a type must
be a complete declaration, and must therefore include at least one access specifier.
Whether a property is hidden or overridden in a derived class, property look-up is
always static. That is, the declared (compile-time) type of the variable used to identify
an object determines the interpretation of its property identifiers. Hence, after the
following code executes, reading or assigning a value to MyObject.Value invokes
Method1 or Method2, even though MyObject holds an instance of TDescendant. But you