Accessing Subtypes

In the majority of code written using Lpp, Lpp objects appear as type let. All operations within Lpp objects are done with accessors. When defining new Lpp objects the user needs to get at the internals of such objects for defining his own accessors. The Lpp the macro makes it easy and safe to access the internals of an Lpp object.

the name exp

Macro

theOrNil name exp

Macro

asThe name exp

Macro

fromThe name exp

Macro

The argument name names some subtype of the Lpp type hierarchy. Lpp takes the liberty to adapt the Common Lisp the construct. Much like Common Lisp the Lpp the macro evaluates the given expression exp and returns the value if it is of the type named by the given name and triggers an exception if the type is not of the given name. In addition the Lpp the macro returns a pointer to that object of type name name. This is a specific pointer of that name type as opposed to the more general Lpp let type. For example:

     classL(MyType) {
         int slot1;
         public: friend void setSlot1(let, int);};
     setSlot1(makeInstance(MyType), 25);
     
     void setSlot1(let x, int val) {    // x is input as general let type
         MyType* mt = the(MyType, x);   // mt is now a specific pointer
         mt->slot1 = val;}              // to a MyType object
     

The the macro enforces that the given object is of type type by doing a dynamic type check. Dynamic type checking can be turned off per compilation unit, See Type Checking.

The asThe macro is the same as the but never does any type checking. It is useful when a function absolutely knows what the type of an Lpp object is. The fromThe macro is the same as asThe but instead demands that the object is of the given type or derived from a supertype of the given type. It is useful when a function absolutely knows what the type or supertype of an Lpp object is. The theOrNil macro is the same as the but also allows nil to pass as the type.