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C++ Class Frameworks and the Visual Component Model
TRect rect;
rect->left = 20;
rect->top = 20;
rect->right = 220;
rect->bottom = 220;
OWL also has a TRect class. In OWL you can create a TRect object and supply the top, left,
bottom, and right members through the constructor:
TRect rect(20, 20, 220, 220);
Obviously, specifying the rectangle’s parameters in the constructor saves you some typing.
In addition, the OWL version of TRect has several constructors to create TRect instances in
different ways. MFC’s CRect class has basically the same type of constructors as OWL’s TRect.
What this means is that in some cases you have to give up the flexibility that C++ allows you
when constructing VCL classes. It’s a minor sacrifice in most cases.
VCL Does Not Have Default Parameters for Functions
To illustrate, let’s examine the Windows API function MessageBox(). This function takes
four parameters: the window handle of the window that is displaying the message box, the
message box text, the message box title, and a Flags parameter that controls which buttons
and icons are displayed on the message box. In MFC or OWL, you can call a message box
by just specifying the message box text:
MessageBox(“This is a message.”);
This is possible because the OWL and MFC versions of MessageBox() have default
parameters for the message box title and style flags. You can specify the additional parameters,
but if you don’t, the default values will be used. This is convenient for throwing up message
boxes with a minimum of fuss. Here is the VCL equivalent to the previous line:
Application->MessageBox(“This is a message”, “Message”, MB_OK);
Because VCL does not have default parameters, you have to specify all the parameters. It’s
not convenient, but not the end of the world, either. Note that in all three cases (MFC, OWL,
and VCL), the framework takes care of supplying the window handle parameter.
VCL Classes Do Not Support Multiple Inheritance
This means that you cannot create a new component derived from two existing components.
I don’t see this as a serious restriction because multiple inheritance is not widely used. Regular
C++ classes that you write for use in your C++Builder applications may use multiple
inheritance.
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