The prototype declaration of a class is as follows:
Remark: In MacPas mode, the Object keyword is replaced by the class keyword for compatibility with other pascal compilers available on the Mac. That means that in MacPas mode, the reserved word ’class’ in the above diagram may be replaced by the reserved word ’object’.
In a class declaration, as many private, protected, published and public blocks as needed can be used: the various blocks can be repeated, and there is no special order in which they must appear.
Methods are normal function or procedure declarations. As can be seen, the declaration of a class is almost identical to the declaration of an object. The real difference between objects and classes is in the way they are created (see further in this chapter).
The visibility of the different sections is as follows:
In the syntax diagram, it can be seen that a class can list implemented interfaces. This feature will be discussed in the next chapter.
Classes can contain Class methods: these are functions that do not require an instance. The Self identifier is valid in such methods, but refers to the class pointer (the VMT).
Remark: Like with functions and pointer types, sometimes a forward definition of a class is needed. A class forward definition is simply the name of the class, with the keyword Class, as in the following example:
When using a class forward definition, the class must be defined in the same unit, in the same section (interface/implementation). It must not necessarily be defined in the same type section.
It is also possible to define class reference types:
Class reference type
Class reference types are used to create instances of a certain class, which is not yet known at compile time, but which is specified at run time. Essentially, a variable of a class reference type contains a pointer to the definition of the speficied class. This can be used to construct an instance of the class corresponding to the definition, or to check inheritance. The following example shows how it works:
This function can be passed a class reference of any class that descends from TComponent. The following is a valid call:
On return of the CreateComponent function, C will contain an instance of the class TEdit. Note that the following call will fail to compile:
because TStream does not descend from TComponent, and AClass refers to a TComponent class. The compiler can (and will) check this at compile time, and will produce an error.
References to classes can also be used to check inheritance:
The above example will raise an exception if the passed instance is not a descendent of TMinClass or a descendent of TMaxClass.
More about instantiating a class can be found in section 6.3, page 297.