Re: Write Function Pointer Array

James Kanze <>
Sun, 29 Jun 2008 02:42:20 -0700 (PDT)
On Jun 29, 1:20 am, Immortal Nephi <> wrote:

On Jun 28, 1:39 pm, James Kanze <> wrote:

On Jun 28, 6:55 pm, Immortal Nephi <> wrote:

On Jun 28, 2:23 am, James Kanze <> wrote:

On Jun 28, 3:07 am, Immortal Nephi <> wro=


     I would like to design an object using class. How can this =


contain 10 member functions. Put 10 member functions into member
function pointer array.
     One member function uses switch to call 10 member functions.=


switch be replaced to member function pointer array?

Yes, but why? What controls the choice? On the whole member
function pointers are awkward and complex; a number of otherwise
competent C++ programmers seem to have problems with them,
because of the syntax. I use them from time to time, but my
experience has been that as the class evolves, they end up being
replaced by "agents", small stand-alone objects which call the
function (and possibly contain additional data).

Thank you very much for your assistance. Yes, you do answer
my question right. Can you please describe what you mean

Small stand-alone objects, which (probably) call the funnction
actually desired. The advantage of this, as opposed to pointers
to member functions, is that the objects can contain additional
data. Thus, you can add cases where two keys call the same
function, but with different arguments.

Agents will generally all derive from a common abstract base
class, with a single function which performs the desired
operation. The array will be of pointers to this base class.
It requires a little bit more typing to set up, but it's a lot
more flexible, and for most C++ programmers that I've
encountered, a lot more readable.

If you want to use small stand-alone object without using
class, you can only create one object at this time. The
flexibaility is less on C style language.

Class is ideal to show multiple objects. You can use class to
define more than one object. Think of class CPU { public: }.
Create an array of four CPU objects. You can expect four CPU
objects to run at the same time like four emulators are

If you have an array, you have multiple objects. All we're
talking about is the type of the object.

Of course, with agents, you do introduce an additional level of
indirection; the array contains pointers to objects, and not the
objects themselves, since we need pointers for polymorphism to
work. Which is the additional typing I referred to. It pays
off in added flexibility, however---and to top it off, it's
usually a bit faster.

Member Function Pointer Array is ideal for CPU's Opcode
emulator like dispatch() or fetch().

Anyway, supposing the calling function receives an int:
    MyClass::dispatch( int key )
        static void (MyClass::* const table[])() =
            // ...
        } ;
        assert( key >= 0 && key < size( table ) ) ;
        (this->*table[ key ])() ;

dispatch() function as above makes sense. Many C programmers
tell that C++ class is slower. They want to stick global
variable and global functions for performance reason.
According to my research, global functions have *direct*
memory. The member functions have *indirect* memory. It
means that member function must receive memory address from
*this pointer* before member function can be called.

C++ programmers wants to convert global variable and
global function into C++ class. They say that C++ class
is about 25% slower performance than C style source code
because extra instruction needs to access indirection. It
is the time to start designing CPU's opcode emulator on
C++ class. They expect faster Intel / AMD CPUs or other
machines with the speed of 4 GHz and beyond.

I'm not really sure I understand. An emulator, today,
doesn't emulate single instructions; it emulates blocks of
code. And its speed is determined by the size of the
blocks, and how effectively it can deduce the actual
semantics of the block, and convert that into native code.

I am sorry that you misunderstand. I do not talk about
emulator. I talk about indirect memory and direct memory.
C++ Compiler translates class and *this pointer* into native
x86 machine language. You can take a look at disassembler.
You will see *lea instruction* and *mov instruction*.

Global variables and global functions use *direct memory* so
you can see *mov instruction*. Member variables and member
functions inside class use *this pointer*. The class needs to
read *this pointer* before it can access member variable or
member function.

With any decent compiler, the this pointer will be in memory,
and on most machines, a based access will be at least as fast,
if not faster, than a direct access. At least on my Sun Sparc,
accessing member variables is faster than accessing global
variables. (The Intel architecture is a bit hindered here by a
lack of registers, so most compilers will only put the this
pointer into a register when optimization is turned on. On the
other hand, no one is going to be using the Intel architecture
if performance is an issue.)

*lea instruction* is the *this pointer* and *mov instruction*
is the member variable or member function.

I recall that class is 25% performance slower than C style
language because it has extra instruction to be fetched like
lea instruction.

You recall wrong. If two programs do the same thing, and
performance is an issue, C++ always wins (compared to C, at
least), because of better encapsulation.

James Kanze (GABI Software)
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