Function lookup tables?

WW · Jul 19 '05, 06:45 PM

Re: Function lookup tables?

John Collyer wrote:[color=blue]
> 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.[/color]

How about a static (initialized) array of function pointers? Or - depending
on what you do - OO and inheritance (vtable)?

--
WW aka Attila

**Victor Bazarov** · Jul 19 '05, 06:45 PM

Re: Function lookup tables?

"John Collyer" <johncollyer@zo ominternet.net> wrote...[color=blue]
> 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?[/color]

You would use an array of pointers to functions. If all of them
have the same type (number, order, and types of arguments and the
return value types), of course.
[color=blue]
> 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.[/color]

#include <iostream>

void print3(int i) {
std::cout << " " << i << std::endl;
}

void print5(int i) {
std::cout << " " << i << std::endl;
}

void print7(int i) {
std::cout << " " << i << std::endl;
}

int main() {
void (*FUNCTION[])(int) = { print3, print5, print7 };
for (int i = 0; i < 20; ++i)
FUNCTION[i % 3](i); // call from a table
return 0;
}

Victor

**Peter van Merkerk** · Jul 19 '05, 06:45 PM

Re: Function lookup tables?

"John Collyer" <johncollyer@zo ominternet.net> wrote in message
news:3f7ee0db$1 _4@corp.newsgro ups.com...[color=blue]
> 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.[/color]

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.merke rk(at)dse.nl

**Gianni Mariani** · Jul 19 '05, 06:45 PM

Re: Function lookup tables?

John Collyer wrote:[color=blue]
> 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.
>[/color]

There are posts that indicate how to do what you're looking for and
they're fast.

However, consider that C++ has another mechanism that is more extensible
and this may solve you problem more effectively ( I don't know, for sure
- it's just an idea spawned by problems I had with function tables.)

C++ allows you to have polymorphic objects - i.e.

struct Poly
{
virtual void WantACracker( Cracker & food ) = 0;

virtual void Say( std::string & words ) = 0;
};

struct Poicephalus : Poly
{
virtual void WantACracker( Cracker & food )
{
if ( food != SunflowerSeed )
{
Say( "YUCK" );
}
}
};

struct Psittacus : Poly
{

Poly()
: fed_count(0)
{
}

int fed_count; // state associated with this Poly

virtual void WantACracker( Cracker & food )
{
if ( food != Ritz )
{
Say( "YUCK" );
}
fed_couint ++;
}
};

Poly * Parrots[] = { new Poicephalus, new Psittacus, new Psittacus };

int main()
{
Parrots[ 1 ]->WantACracker ( Ritz );

Parrots[ 0 ]->WantACracker ( SunflowerSeed );
}

The cost of the virtual overhead depends greatly on how you use it but
it is usually no more than 2 indirections and a call on most compilers.
(compared to one indirection and call with a function table). However
the flexibilty of having state associated with the function is usually
far more of a performance advantage than you're giving up.

**Ron Ruble** · Jul 19 '05, 06:47 PM

Re: Function lookup tables?

"Peter van Merkerk" <merkerk@deadsp am.com> wrote in message news:blmocb$dpf hf$1@ID-133164.news.uni-berlin.de...
<snip>[color=blue]
> Note that in C++ polymorphism might be a more appropriate solution than
> using function pointers or switches.[/color]

Or might not. This kind of thing is particularly useful when the
decision about which function should be used isn't made at
compile time.

**Gianni Mariani** · Jul 19 '05, 06:47 PM

Re: Function lookup tables?

Ron Ruble wrote:[color=blue]
> "Peter van Merkerk" <merkerk@deadsp am.com> wrote in message news:blmocb$dpf hf$1@ID-133164.news.uni-berlin.de...
> <snip>
>[color=green]
>>Note that in C++ polymorphism might be a more appropriate solution than
>>using function pointers or switches.[/color]
>
>
> Or might not. This kind of thing is particularly useful when the
> decision about which function should be used isn't made at
> compile time.[/color]

How does the "decision not at compile time" change the polymorphic
suggestion ?

**Ron Ruble** · Jul 19 '05, 06:47 PM

Re: Function lookup tables?

"Gianni Mariani" <gi2nospam@mari ani.ws> wrote in message news:blnj37$brd @dispatch.conce ntric.net...[color=blue]
> Ron Ruble wrote:[color=green]
> > Or might not. This kind of thing is particularly useful when the
> > decision about which function should be used isn't made at
> > compile time.[/color]
>
> How does the "decision not at compile time" change the polymorphic
> suggestion ?[/color]

Forgive me, I realize now that I didn't state my thoughts
clearly.

I was thinking of several apps I've worked on where
specific operations were performed based on interpreting
commands in a text file, effectively a user scripted
operation, using functions in a module loaded at
run-time.

In these cases, the -logical- operation was based on
the index into a table of function pointers, where the
index had a meaning known to the programmer
(startup code, load file, shutdown, etc.).

The actual function the pointer was set to was provided
by the dynamic module at run time, based on values
in a text file. Certainly, there are other ways to do this,
including some methods using polymorphism.

But in the specific cases I was thinking of, using a table
of function pointers was clean and simple.

Not that a polymorphic solution would be bad, just
that the table of function pointers was perfectly
adequate, and in the cases I was thinking of , it
eliminated additional design effort.

**Gianni Mariani** · Jul 19 '05, 06:47 PM

Re: Function lookup tables?

Ron Ruble wrote:[color=blue]
> "Gianni Mariani" <gi2nospam@mari ani.ws> wrote in message news:blnj37$brd @dispatch.conce ntric.net...
>[color=green]
>>Ron Ruble wrote:[/color][/color]
....[color=blue]
>
> Not that a polymorphic solution would be bad, just
> that the table of function pointers was perfectly
> adequate, and in the cases I was thinking of , it
> eliminated additional design effort.[/color]

I've implemented both function table and polymorphic approaches in a
scenario like this one and I have found the polymorphic system to be
more easily extensible and hence in general less design effort.

I suppose we can chalk this off to different experience.

G

**John Collyer** · Jul 19 '05, 06:47 PM

Re: Function lookup tables?

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}
}
myclass::~mycla ss()
{
}

char myclass::F0(siz e_t address)
{
}

char myclass::F1(siz e_t address)
{
}

char myclass::F2(siz e_t address)
{
}

char myclass::F3(siz e_t address)
{
}

char myclass::F4(siz e_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@deadsp am.com> wrote in message
news:blmocb$dpf hf$1@ID-133164.news.uni-berlin.de...[color=blue]
> "John Collyer" <johncollyer@zo ominternet.net> wrote in message
> news:3f7ee0db$1 _4@corp.newsgro ups.com...[color=green]
> > 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.[/color]
>
> 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.merke rk(at)dse.nl
>
>[/color]

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**John Collyer** · Jul 19 '05, 06:47 PM

Re: Function lookup tables?

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

"John Collyer" <johncollyer@zo ominternet.net> wrote in message
news:3f802381$1 _4@corp.newsgro ups.com...[color=blue]
> The c code for function lookup table works great, but?
> I have the lookup table in a class as a member of that class.
> I want to initialize the function lookup table array to my member
> function calls of the class inside of the constructor for the class.
> What is the correct code to initialize the function pointer array
> in the constructor? I have:
>
> inside the class header file
>
> typedef void(*func_ptr) (); // declare pointer to function
>
> class myclass
> {
> public:
> myclass();
> virtual ~myclass();
>
> func_ptr Functions[5]; // declare the function pointer array
>
> char F0(size_t address); // member functions
> char F1(size_t address);
> char F2(size_t address);
> char F3(size_t address);
> char F4(size_t address);
>
> };
>
> Inside the class source cpp file
>
> myclass:myclass ()
> {
> Functions[] = { F0,F1,F2,F3,F4} ; // semi colen is there typo
> }
> myclass::~mycla ss()
> {
> }
>
> char myclass::F0(siz e_t address)
> {
> }
>
> char myclass::F1(siz e_t address)
> {
> }
>
> char myclass::F2(siz e_t address)
> {
> }
>
> char myclass::F3(siz e_t address)
> {
> }
>
> char myclass::F4(siz e_t address)
> {
> }
>
> The actual function pointer lookup table array is 256 functions or[/color]
pointers[color=blue]
> to functions, so the initialization list inside the constructor is quite
> long and uses more then one line. Everything works except I get errors[/color]
with[color=blue]
> the array initialization list in the constructor; errors dealing with the
> initialation of the array.
>
> syntax error : ']'
> error C2143: syntax error : missing ';' before '{'
> error C2143: syntax error : missing ';' before '}'
> error C2059: syntax error : ']'
> syntax error : missing ';' before '{'
> error C2143: syntax error : missing ';' before '}'
>
> Any help?
>
> John Collyer
>
> "Peter van Merkerk" <merkerk@deadsp am.com> wrote in message
> news:blmocb$dpf hf$1@ID-133164.news.uni-berlin.de...[color=green]
> > "John Collyer" <johncollyer@zo ominternet.net> wrote in message
> > news:3f7ee0db$1 _4@corp.newsgro ups.com...[color=darkred]
> > > 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[/color][/color][/color]
tables.[color=blue][color=green][color=darkred]
> > > If anyone could help I would appreciate it.[/color]
> >
> > 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[/color][/color]
resulting[color=blue][color=green]
> > 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.merke rk(at)dse.nl
> >
> >[/color]
>
>
>
>
> -----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
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**Julián Albo** · Jul 19 '05, 06:47 PM

Re: Function lookup tables?

John Collyer escribió:
[color=blue]
> typedef void(*func_ptr) (); // declare pointer to function
>
> class myclass
> {
> public:
> myclass();
> virtual ~myclass();
>
> func_ptr Functions[5]; // declare the function pointer array[/color]

Make Functions static.

static func_ptr Functions [5];

And intialize it in the .cpp file outside any function as:

func_ptr myclass::Functi ons []= { F0, F1, F2, F3, F4 };

Regards.

Function lookup tables?

Function lookup tables?

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