How to pass a function name as an argument?

[Yangang Bao]

I have a simple test program. It doesn't work. I want to know what is the
reason and how to
fix it. Maybe I should use template function to do it. But I don't know how.

Here is the simple program. Strange enough that the problem never happens in
C languge,
i.e., in the program, if I call "foo2", it works fine. Why "foo1" doesn't
work?

Thanks for your answers.
////////////////////////////////////////////////////////////////////////////
/
file://main.cpp
////////////////////////////////////////////////////////////////////////////
/
#include <stdio.h>
#include <stdlib.h>
#include "Funct.h"

void main()
{
double s;
Funct funct;
s = funct.drive();
printf("Result= %lf", s);
}

////////////////////////////////////////////////////////////////////////////
/
file://Funct.h
////////////////////////////////////////////////////////////////////////////
/
double foo2(double t);

class Funct
{
double Integrate(doubl e(*func)(double ), double a, double b);
double foo1(double t);

public:
double drive();

};
////////////////////////////////////////////////////////////////////////////
/
file://Funct.cpp
////////////////////////////////////////////////////////////////////////////
/
#include "Funct.h"

double Funct::Integrat e(double (*func)(double) , double a, double b)
{
double c = a+b;
return func(c);
}

double Funct::foo1(dou ble t)
{
return t*t;
}

double Funct::drive()
{
double t = Integrate(foo1, 2, 3);
// double t = Integrate(foo2, 2, 3);
return t;
}

double foo2(double t)
{
return t*t;
}
*************** *************** ********

[Josephine Schafer]

Re: How to pass a function name as an argument?


"Yangang Bao" <ybao@ualberta. ca> wrote in message
news:bmdice$i1l $1@tabloid.srv. ualberta.ca...[color=blue]
> I have a simple test program. It doesn't work. I want to know what is the
> reason and how to
> fix it. Maybe I should use template function to do it. But I don't know how.
>
> Here is the simple program. Strange enough that the problem never happens in
> C languge,
> i.e., in the program, if I call "foo2", it works fine. Why "foo1" doesn't
> work?
>
> Thanks for your answers.
> ////////////////////////////////////////////////////////////////////////////
> /
> file://main.cpp
> ////////////////////////////////////////////////////////////////////////////
> /
> #include <stdio.h>
> #include <stdlib.h>
> #include "Funct.h"
>
> void main()
> {
> double s;
> Funct funct;
> s = funct.drive();
> printf("Result= %lf", s);
> }
>
> ////////////////////////////////////////////////////////////////////////////
> /
> file://Funct.h
> ////////////////////////////////////////////////////////////////////////////
> /
> double foo2(double t);
>
> class Funct
> {
> double Integrate(doubl e(*func)(double ), double a, double b);
> double foo1(double t);
>
> public:
> double drive();
>
> };
> ////////////////////////////////////////////////////////////////////////////
> /
> file://Funct.cpp
> ////////////////////////////////////////////////////////////////////////////
> /
> #include "Funct.h"
>
> double Funct::Integrat e(double (*func)(double) , double a, double b)
> {
> double c = a+b;
> return func(c);
> }
>
> double Funct::foo1(dou ble t)
> {
> return t*t;
> }
>
> double Funct::drive()
> {
> double t = Integrate(foo1, 2, 3);
> // double t = Integrate(foo2, 2, 3);
> return t;
> }
>
> double foo2(double t)
> {
> return t*t;
> }[/color]

See the first parameter of Integrate.
It is a pointer to a function that takes a double as parameter and returns a
double.
What is the type of foo1?
It is double (Funct::*)(doub le).
Pointers to non-static member functions are not type compatible with regular
pointers to functions, hence the error.

HTH,
J.Schafer

[John Carson]

Re: How to pass a function name as an argument?

"Yangang Bao" <ybao@ualberta. ca> wrote in message
news:bmdice$i1l $1@tabloid.srv. ualberta.ca[color=blue]
> I have a simple test program. It doesn't work. I want to know what is
> the reason and how to
> fix it. Maybe I should use template function to do it. But I don't
> know how.
>
> Here is the simple program. Strange enough that the problem never
> happens in C languge,[/color]

The C language doesn't have member functions, so the issue never arises.
[color=blue]
> i.e., in the program, if I call "foo2", it works fine. Why "foo1"
> doesn't work?[/color]

Because it is a member function, not a "free" function. The syntax for
pointers to member functions is very different to the syntax for pointers to
ordinary functions.

[snip]
[color=blue]
>
> class Funct
> {
> double Integrate(doubl e(*func)(double ), double a, double b);[/color]

// Try the following overload:

double Integrate(doubl e(Funct::*func) (double), double a, double b);

// which you define as

double Funct::Integrat e(double (Funct::*func)( double), double a, double b)
{
double c = a+b;
return (this->*func)(c);
}


--
John Carson
1. To reply to email address, remove donald
2. Don't reply to email address (post here instead)

[Yangang Bao]

Re: How to pass a function name as an argument?

It seems it works. You are really helpful. Thanks a lot.

[Yangang Bao]

Re: How to pass a function name as an argument?

I tried to adopt this method in my program. And unfortunately a tragedy
happens. The problem is that I have to pass the function name as argument
twice.
And the VC6.0 compiler still shows the wrong message: INTERNAL COMPILER
ERROR. I just wonder how other guys implemented the quadrature
functions in C++.

Here is the simple test program:
//////////////////////////////////////////////////////////////////////////
// main.cpp
//////////////////////////////////////////////////////////////////////////
#include <stdio.h>
#include <stdlib.h>

#include "Funct.h"

void main()
{
Funct funct;
funct.drive();
}

//////////////////////////////////////////////////////////////////////////
// Funct.h
//////////////////////////////////////////////////////////////////////////
#ifndef FUNCT_INCLUDED_
#define FUNCT_INCLUDED_

class Funct
{
public:
double Quadrature(doub le (Funct::*func)( double), double a, double b);
double QuadStep(double (Funct::*func)( double), double c, double d);

double foo1(double t);

double drive();
};

#endif

//////////////////////////////////////////////////////////////////////////
// Funct.cpp
//////////////////////////////////////////////////////////////////////////
#include "Funct.h"

double Funct::drive()
{
double a = 1;
double b = 2;
double s;

s = Quadrature(foo1 , a, b);
return s;

}

double Funct::foo1(dou ble t)
{
return t*t;
}

double Funct::Quadratu re(double (Funct::*func)( double), double a, double b)
{
double c= (a+b)/2.0;
double s1, s2;

s1 = QuadStep((this->*func), a, c);
s2 = QuadStep((this->*func), c, d);
return s;
}

double Funct::QuadStep (double (Funct::*func)( double), double c, double d)
{
return (this->*func)(c*d);
}


"Yangang Bao" <ybao@ualberta. ca> wrote in message
news:bmfr0v$le9 $1@tabloid.srv. ualberta.ca...[color=blue]
> It seems it works. You are really helpful. Thanks a lot.
>
>[/color]

[tom_usenet]

Re: How to pass a function name as an argument?

On Tue, 14 Oct 2003 12:10:35 -0600, "Yangang Bao" <ybao@ualberta. ca>
wrote:
[color=blue]
>I tried to adopt this method in my program. And unfortunately a tragedy
>happens. The problem is that I have to pass the function name as argument
>twice.
>And the VC6.0 compiler still shows the wrong message: INTERNAL COMPILER
>ERROR. I just wonder how other guys implemented the quadrature
>functions in C++.[/color]
[color=blue]
>double Funct::Quadratu re(double (Funct::*func)( double), double a, double b)
>{
> double c= (a+b)/2.0;
> double s1, s2;
>
> s1 = QuadStep((this->*func), a, c);
> s2 = QuadStep((this->*func), c, d);[/color]

The above two should be:

s1 = QuadStep(func, a, c);
s2 = QuadStep(func, c, d);

You only dereference the pointer to member when calling it.
[color=blue]
> return s;
>}
>
>double Funct::QuadStep (double (Funct::*func)( double), double c, double d)
>{
> return (this->*func)(c*d);[/color]

That's fine, since you're calling it.

Tom