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| <h3 class="section">8.11 Virtual Methods</h3> |
| |
| <p><< The following examples are based on a4.C >> |
| |
| <p>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: |
| |
| <pre class="example"> class A { |
| public: |
| int Adat; |
| virtual int A_virt (int arg) { return arg; }; |
| }; |
| </pre> |
| <p>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 ‘<samp><span class="samp">Adat</span></samp>’, an integer, starting at structure offset 0 and |
| occupying 32 bits. |
| |
| <p>The second field in the class struct is not explicitly defined by the |
| C<tt>++</tt> 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 ‘<samp><span class="samp">$vf</span></samp>’ and continues with a |
| type reference to the class it is part of. In this example the type |
| reference for class A is 20 so the name of its vtable pointer field is |
| ‘<samp><span class="samp">$vf20</span></samp>’, followed by the usual colon. |
| |
| <p>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). |
| |
| <p>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<tt>++</tt> |
| type definitions, as shown earlier. |
| |
| <p>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. |
| |
| <p>Next is the method definition for the virtual member function <code>A_virt</code>. |
| Its description starts out using the same format as the non-virtual |
| member functions described above, except instead of a dot after the |
| ‘<samp><span class="samp">A</span></samp>’ 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. |
| |
| <p>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. |
| |
| <p>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). |
| |
| <p>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. |
| |
| <p>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 ‘<samp><span class="samp">~%</span></samp>’ and followed by a final semi-colon. |
| |
| <pre class="display"> .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 |
| </pre> |
| <!-- FIXME: bogus line break. --> |
| <pre class="example"> .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 |
| </pre> |
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