Type Name Declaration Descriptors

Type Name Declaration Descriptors

The first group includes all descriptors for names associated with types. This includes struct type descriptors, array type descriptors and a special descriptor for the built-in integer type. The definitions for this group of type descriptors is shown in figure *.

    /* Structure used to reference common fields of type descriptor.    */
struct unktypedesc {
   COMMONFIELDS
   int typesize;                  /* Memory required for var. of this type */
 };


    /* Structure used for array type declaration descriptors.     */
struct arraydesc {
   COMMONFIELDS
   int typesize;                /* Memory required for array of this type */
   int size;                    /* Number of elements in array            */
   union dcldesc * elmntvar;    /* decl. descriptor for imaginary variable of
   	                                   this array's element type      */
 };


    /* Structure used for struct type declaration descriptors.      */
struct structdesc {
   COMMONFIELDS
   int typesize;                /* Memory required for recd. of this type    */
   union dcldesc *fields;       /* Header for list of this type's components */
 };

All type name declaration descriptors share a field named typesize which should be set equal to the number of units of memory needed to hold an element of the type. To provide a way to write clear code that accesses this field without determining the particular sort of type involved, a structure named unktypedesc is declared and a choice of unktype is included in the decldesc union type.

Structure types are described by declaration descriptors of type structdesc. The only special component of such a descriptor is a pointer, fields, to a list of declaration descriptors for the components of the structure type.

Array types are described by declaration descriptors of type arraydesc. An array declaration descriptor contains a size field in which the compiler will store the number of elements in the array. It also needs to somehow represent the element type of the array. The natural way to accomplish this would be to have the array type's declaration descriptor point to the descriptor for the element type. We will use a somewhat more complex scheme (for reasons that will not become clear until near the end of the project). For each array you will need to create a declaration descriptor for an imaginary variable of the array's element type. The array type's elmntvar field will then be set to point to this variable descriptor which will in turn point to the descriptor for the array's element type.

Type Name Declaration Descriptors