Teach Yourself Borland C++ in 14 Days - portal

494 Day 14
This declaration tells the compiler to call the GetColor() method of the class (the read
method) when the Color property is read, and to call the SetColor() method of the class (the
write method) when the Color property is written to. The exact implementation of the
GetColor() and SetColor() methods is entirely up to the component writer.
Properties can use direct access rather than read or write methods. A property almost always
has a private class data member that is used to store the actual value of the property. It is
common to use a write method to effect some change in the component when the property
is written to, but no such method when the property is read. In that case you would specify
a write method for the property but use direct access to read the property directly from the
class data member that holds the property’s value. In that case the property’s declaration
would look something like this:
__property int Color { read=FColor, write=SetColor };
Now when the property is written to, the SetColor() method is called as before, but when
the property is read, the value of the FColor variable is returned rather than a read method
being called. (The FColor variable would have been declared in the private section of the
component’s class declaration.)
A property need not declare either a read or write specifier. If no write specifier is included,
the property will be a read-only property (a valuable option at times). If no read specifier is
included, the property will be a write-only property (of dubious value). If neither a read nor
a write specifier are included, the property can neither be read nor written to, making it a
worthless property.
Default Values
Properties can specify the default value that the property should have when it is displayed in
the Object Inspector. For instance, let’s say that your Color property should default to white
(clWhite). In that case, the declaration of the property would look like this:
__property int Color { read=FColor, write=SetColor, default=clWhite };
This tells the Form Designer to display the default value clWhite in the Object Inspector next
to the property. Note that this does not automatically set the underlying data member’s value
to clWhite. You must still set the underlying data member to the value clWhite in the
component’s constructor. The use of the default specifier is optional. If you do not specify
a default value, 0 will be used (or false in the case of bool properties) as the default value.
Storing the Property
The stored specifier controls how a property is stored in a form’s form file. Storing the
property refers to storing the property’s value in the form file when the project is saved. The