Virtual Tables Example

Simple language that understands subtyping and virtual dispatch (vtables). Syntax is below:

i is an integer
x is a variable
cname is a class name
mname is a method name

exp ::= i | lhs
lhs ::= x | lhs.x | this
type ::= int | cname
vdec ::= type x
stmt ::= vdec = new cname(exp*) |
         vdec = exp.mname(exp*) |
         print(exp) |
         return exp |
         lhs = exp |
         super(exp*) |
         stmt; stmt |
         empty
mdef ::= [virtual] type mname(vdec*) { stmt }
cdef ::= class cname [extends cname ] {
           vdec*
           init(vdec*) { stmt }
           mdef*
         }
program ::= cdef* stmt

Intentional Limitations

• Most useful kinds of expressions and statements are missing, to dramatically simplify the language. See other examples in the organization for how to handle those.
• Most expressions do not nest (e.g., new and method calls are statements, not expressions). This is to simplify codegen so that every expression can be compiled with a single register, without needing the stack.
• Overriding methods must be marked virtual, unlike C++
• There is no lexer or parser.
• The typechecker is not well-tested.