pointer to pure virual function

[Klaas Vantournhout]

Hi,

I try to set a function pointer to a pure virtual functions in the base
class itself.

To shorten, assume the following code

class base {
public:
base() { base::Fptr = &base::F;}
void (*Fptr) (int);
virtual void F(int) = 0;
};

compiling it gives me

[~]$ g++ -c virtualptr.cpp
virtualptr.cpp: In constructor ‘base::base()’:
virtualptr.cpp:3: error: cannot convert ‘void (base::*)(int)’ to ‘void
(*)(int)’ in assignment

Where am I going wrong, and more important.
Is it actually possible?

[Victor Bazarov]

Klaas Vantournhout wrote:

I try to set a function pointer to a pure virtual functions in the
base class itself.

To shorten, assume the following code

class base {
public:
base() { base::Fptr = &base::F;}
void (*Fptr) (int);

You need to declare Fptr as

void (base::*Fptr)(int);

virtual void F(int) = 0;
};

compiling it gives me

[~]$ g++ -c virtualptr.cpp
virtualptr.cpp: In constructor 'base::base()':
virtualptr.cpp:3: error: cannot convert 'void (base::*)(int)' to 'void
(*)(int)' in assignment

Where am I going wrong, and more important.

Read the error message.

Is it actually possible?

Should be possible. What's the point, though? You can't call it.

V
--
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[Klaas Vantournhout]

Klaas Vantournhout wrote:

Hi,

I try to set a function pointer to a pure virtual functions in the base
class itself.

To shorten, assume the following code

class base {
public:
base() { base::Fptr = &base::F;}
void (*Fptr) (int);
virtual void F(int) = 0;
};

compiling it gives me

[~]$ g++ -c virtualptr.cpp
virtualptr.cpp: In constructor ‘base::base()’:
virtualptr.cpp:3: error: cannot convert ‘void (base::*)(int)’ to ‘void
(*)(int)’ in assignment

Where am I going wrong, and more important.
Is it actually possible?

Okay compilation is successful when implementing the following

class base {
public:
base() { base::Fptr = &base::F;}
void (base::*Fptr) (int);
virtual void F(int) = 0;
};

never second-guess the compiler ;-)

[Salt_Peter]

Klaas Vantournhout wrote:

Hi,

I try to set a function pointer to a pure virtual functions in the base
class itself.

To shorten, assume the following code

class base {
public:
base() { base::Fptr = &base::F;}
void (*Fptr) (int);
virtual void F(int) = 0;
};

compiling it gives me

[~]$ g++ -c virtualptr.cpp
virtualptr.cpp: In constructor 'base::base()':
virtualptr.cpp:3: error: cannot convert 'void (base::*)(int)' to 'void
(*)(int)' in assignment

Where am I going wrong, and more important.
Is it actually possible?

Yes, but what for since you can't use it. Note that nothing stops from
calling the pure virtual from derived.

#include <iostream>

class base {
public:
base() { }
virtual void F(int) = 0;
};

void base::F(int n) { std::cout << "base::F(int)\n"; }

class derived : public base
{
void F(int n)
{
std::cout << "derived::F(int)\n";
base::F(n);
}
};

int main()
{
derived d;
base* p_base = &d;
p_base->F(0);
}

/*
derived::F(int)
base::F(int)
*/

[Pete Becker]

Victor Bazarov wrote:

>
Should be possible. What's the point, though? You can't call it.

You can call it. It's just an ordinary virtual call:

#include <iostream>
struct Base
{
virtual void f() = 0;
virtual ~Base() {}
};

struct Derived : public Base
{
void f() { std::cout << "Here I am!\n"; }
};

int main()
{
void (Base::*fp)() = &Base::f;
Base *bp = new Derived;
(bp->*fp)();
delete bp;
return 0;
}

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)