is at o set 0 (or dword 0) and m2 is at o set 4 (dword 1). The m2 method pointers are the same for the A and B class vtables because class B inherits the m2 method from the A class.

Lines 25 to 32 show how one could call a virtual function by reading its address out of the vtable for the object15. The method address is stored into a C-type function pointer with an explicit this pointer. From the output in Figure 7.25, one can see that it does work. However, please do not write code like this! This is only used to illustrate how the virtual methods use the vtable.

There are some practical lessons to learn from this. One important fact is that one would have to be very careful when reading and writing class variables to a binary file. One can not just use a binary read or write on the entire object as this would read or write out the vtable pointer to the

file! This is a pointer to where the vtable resides in the program’s memory and will vary from program to program. This same problem can occur in C with structs, but in C, structs only have pointers in them if the programmer explicitly puts them in. There are no obvious pointers defined in either the A or B classes.

Again, it is important to realize that di erent compilers implement virtual methods di erently. In Windows, COM (Component Object Model) class objects use vtables to implement COM interfaces16. Only compilers that implement virtual method vtables as Microsoft does can create COM classes. This is why Borland uses the same implementation as Microsoft and one of the reasons why gcc can not be used to create COM classes.

The code for the virtual method looks exactly like a non-virtual one. Only the code that calls it is di erent. If the compiler can be absolutely sure of what virtual method will be called, it can ignore the vtable and call the method directly (e.g., use early binding).

7.2.6Other C++ features

The workings of other C++ features (e.g., RunTime Type Information, exception handling and multiple inheritance) are beyond the scope of this text. If the reader wishes to go further, a good starting point is The Annotated C++ Reference Manual by Ellis and Stroustrup and The Design and Evolution of C++ by Stroustrup.

15Remember this code only works with the MS and Borland compilers, not gcc. 16COM classes also use the stdcall calling convention, not the standard C one.

Appendix A

80x86 Instructions

A.1 Non-floating Point Instructions

This section lists and describes the actions and formats of the non-

floating point instructions of the Intel 80x86 CPU family. The formats use the following abbreviations:

These can be combined for the multiple operand instructions. For example, the format R, R means that the instruction takes two register operands. Many of the two operand instructions allow the same operands. The abbreviation O2 is used to represent these operands: R,R R,M R,I M,R M,I. If a 8-bit register or memory can be used for an operand, the abbreviation, R/M8 is used.

The table also shows how various bits of the FLAGS register are a ected by each instruction. If the column is blank, the corresponding bit is not a ected at all. If the bit is always changed to a particular value, a 1 or 0 is shown in the column. If the bit is changed to a value that depends on the operands of the instruction, a C is placed in the column. Finally, if the bit is modified in some undefined way a ? appears in the column. Because the

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