Re: Function lookup tables?

From: John Collyer (johncollyer_at_zoominternet.net)
Date: 10/05/03 

Date: Sun, 5 Oct 2003 09:58:13 -0400

Ignore any typos the source is good except howto initialize the function
pointer array inside the constructor

"John Collyer" <johncollyer@zoominternet.net> wrote in message
news:3f802381$1_4@corp.newsgroups.com...
> The c code for function lookup table works great, but?
> I have the lookup table in a class as a member of that class.
> I want to initialize the function lookup table array to my member
> function calls of the class inside of the constructor for the class.
> What is the correct code to initialize the function pointer array
> in the constructor? I have:
>
> inside the class header file
>
> typedef void(*func_ptr)(); // declare pointer to function
>
> class myclass
> {
> public:
> myclass();
> virtual ~myclass();
>
> func_ptr Functions[5]; // declare the function pointer array
>
> char F0(size_t address); // member functions
> char F1(size_t address);
> char F2(size_t address);
> char F3(size_t address);
> char F4(size_t address);
>
> };
>
> Inside the class source cpp file
>
> myclass:myclass()
> {
> Functions[] = { F0,F1,F2,F3,F4}; // semi colen is there typo
> }
> myclass::~myclass()
> {
> }
>
> char myclass::F0(size_t address)
> {
> }
>
> char myclass::F1(size_t address)
> {
> }
>
> char myclass::F2(size_t address)
> {
> }
>
> char myclass::F3(size_t address)
> {
> }
>
> char myclass::F4(size_t address)
> {
> }
>
> The actual function pointer lookup table array is 256 functions or
pointers
> to functions, so the initialization list inside the constructor is quite
> long and uses more then one line. Everything works except I get errors
with
> the array initialization list in the constructor; errors dealing with the
> initialation of the array.
>
> syntax error : ']'
> error C2143: syntax error : missing ';' before '{'
> error C2143: syntax error : missing ';' before '}'
> error C2059: syntax error : ']'
> syntax error : missing ';' before '{'
> error C2143: syntax error : missing ';' before '}'
>
> Any help?
>
> John Collyer
>
> "Peter van Merkerk" <merkerk@deadspam.com> wrote in message
> news:blmocb$dpfhf$1@ID-133164.news.uni-berlin.de...
> > "John Collyer" <johncollyer@zoominternet.net> wrote in message
> > news:3f7ee0db$1_4@corp.newsgroups.com...
> > > Hi,
> > >
> > > In assembly language you can use a lookup table to call functions.
> > >
> > > 1. Lookup function address in table
> > > 2. Call the function
> > >
> > > Like:
> > >
> > > CALL FUNCTION[INDEX]
> > >
> > > FUNCTION DD FUNC1, FUNC2, FUNC3
> > >
> > > FUNC1:
> > > FUNC2:
> > > FUNC3:
> > > RET
> > >
> > > How can I do this in C++ I would like it to be reasonably fast also?
> > > The way I am doing it now is with a big switch statement. The switch
> > > uses the index number for lookup. This seems like a bad way to do
> > > what I'd would like. The preferred way would be with the lookup
tables.
> > > If anyone could help I would appreciate it.
> >
> > Try this:
> >
> > #include <iostream>
> > using std::cout;
> >
> > typedef void(*func_ptr)();
> >
> > void func1()
> > {
> > cout << "func1()\n";
> > }
> >
> > void func2()
> > {
> > cout << "func2()\n";
> > }
> >
> > void func3()
> > {
> > cout << "func3()\n";
> > }
> >
> > int main()
> > {
> > func_ptr func[3] = {func1, func2, func3};
> > for(int i = 0; i < 3; ++i)
> > {
> > func[i]();
> > }
> >
> > return 0;
> > }
> >
> > Depending on the optimization capabilities of your compiler the
resulting
> > code may be as fast as the assembler equivalent.
> >
> > Note that in C++ polymorphism might be a more appropriate solution than
> > using function pointers or switches.
> >
> > --
> > Peter van Merkerk
> > peter.van.merkerk(at)dse.nl
> >
> >
>
>
>
>
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