(stabs) Virtual Methods

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(stabs) Inheritance

 
 Virtual Methods
 ===============
 
    << The following examples are based on a4.C >>
 
    The presence of virtual methods in a class definition adds additional
 data to the class description.  The extra data is appended to the
 description of the virtual method and to the end of the class
 description.  Consider the class definition below:
 
      class A {
      public:
              int Adat;
              virtual int A_virt (int arg) { return arg; };
      };
 
    This results in the stab below describing class A.  It defines a new
 type (20) which is an 8 byte structure.  The first field of the class
 struct is `Adat', an integer, starting at structure offset 0 and
 occupying 32 bits.
 
    The second field in the class struct is not explicitly defined by the
 C++ class definition but is implied by the fact that the class contains
 a virtual method.  This field is the vtable pointer.  The name of the
 vtable pointer field starts with `$vf' and continues with a type
 reference to the class it is part of.  In this example the type
 reference for class A is 20 so the name of its vtable pointer field is
 `$vf20', followed by the usual colon.
 
    Next there is a type definition for the vtable pointer type (21).
 This is in turn defined as a pointer to another new type (22).
 
    Type 22 is the vtable itself, which is defined as an array, indexed
 by a range of integers between 0 and 1, and whose elements are of type
 17.  Type 17 was the vtable record type defined by the boilerplate C++
 type definitions, as shown earlier.
 
    The bit offset of the vtable pointer field is 32.  The number of bits
 in the field are not specified when the field is a vtable pointer.
 
    Next is the method definition for the virtual member function
 `A_virt'.  Its description starts out using the same format as the
 non-virtual member functions described above, except instead of a dot
 after the `A' there is an asterisk, indicating that the function is
 virtual.  Since is is virtual some addition information is appended to
 the end of the method description.
 
    The first number represents the vtable index of the method.  This is
 a 32 bit unsigned number with the high bit set, followed by a
 semi-colon.
 
    The second number is a type reference to the first base class in the
 inheritance hierarchy defining the virtual member function.  In this
 case the class stab describes a base class so the virtual function is
 not overriding any other definition of the method.  Therefore the
 reference is to the type number of the class that the stab is
 describing (20).
 
    This is followed by three semi-colons.  One marks the end of the
 current sub-section, one marks the end of the method field, and the
 third marks the end of the struct definition.
 
    For classes containing virtual functions the very last section of the
 string part of the stab holds a type reference to the first base class.
 This is preceded by `~%' and followed by a final semi-colon.
 
      .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
              field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
              field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
              sym_desc(array)index_type_ref(range of int from 0 to 1);
              elem_type_ref(vtbl elem type),
              bit_offset(32);
              meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
              :arg_type(int),protection(public)normal(yes)virtual(yes)
              vtable_index(1);class_first_defining(A);;;~%first_base(A);",
              N_LSYM,NIL,NIL,NIL
 
      .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
              A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
 

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