DJ_AM_Juicebox
asked on
easy function pointer question
Hi,
I have two classes, I want to know how one class can set a pointer to a function to another like:
class A {
void DoSomething();
void SetFunctionPointer(?); { m_pFunctionPointer = ?; }
void* m_pFunctionPointer;
};
class B {
A* m_p;
void SayHello() { cout << "Hello" << endl; }
};
void A::DoSomething()
{
m_pFunctionPointer();
}
B::B()
{
m_p->SetFunctionPointer(&S
}
Thanks
I have two classes, I want to know how one class can set a pointer to a function to another like:
class A {
void DoSomething();
void SetFunctionPointer(?); { m_pFunctionPointer = ?; }
void* m_pFunctionPointer;
};
class B {
A* m_p;
void SayHello() { cout << "Hello" << endl; }
};
void A::DoSomething()
{
m_pFunctionPointer();
}
B::B()
{
m_p->SetFunctionPointer(&S
}
Thanks
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jkr
BTW, see also http://www.newty.de/fpt/index.html ("The Function Pointer Tutorials") for more info.
The problem with member function pointers is that you need both the function pointer and an instance of the class the function is a member from. Because of that there are only a few known senseful samples where any one has used member function pointers, You see it in jkr's sample above. He creates a B object before calling the function pointer but the B is any B and has only default initialization. So that B object cannot do any specific things but only that which are valid for all instances of class B. for that purpose you can use static member functions which can be called without needing an instance of B but only with the class prefix:
class B {
public:
B();
static void SayHello() { cout << "In B: Hello" << endl; }
};
You can call it everywhere by simply
B::SayHello();
And function pointers to static member functions can be treated like function pointers to global functions:
// a typedef makes it simpler to define the type
typedef void (*SayFunc)();
class A
{
SayFunc funcPtr; // we use our typedef here
public:
A(SayFunc sf) : funcPtr(sf) { } // pass it to the constructor
void say() { funcPtr(); }
};
That's all.
Regards, Alex
class B {
public:
B();
static void SayHello() { cout << "In B: Hello" << endl; }
};
You can call it everywhere by simply
B::SayHello();
And function pointers to static member functions can be treated like function pointers to global functions:
// a typedef makes it simpler to define the type
typedef void (*SayFunc)();
class A
{
SayFunc funcPtr; // we use our typedef here
public:
A(SayFunc sf) : funcPtr(sf) { } // pass it to the constructor
void say() { funcPtr(); }
};
That's all.
Regards, Alex
Try this :
#include <iostream>
using namespace std;
class A;
class B {
public:
A* m_p;
B();
void SayHello() { cout << "Hello" << endl; }
~B();
};
class A {
private:
void (B::*m_pFunctionPointer)()
public:
void DoSomething();
void SetFunctionPointer(void (B::*fun_ptr)()) { m_pFunctionPointer = fun_ptr; }
};
void A::DoSomething() {
B b;
(b.*m_pFunctionPointer)();
}
B::B() {
m_p = new A();
m_p->SetFunctionPointer(&B
}
B::~B() {
delete m_p;
}
int main(void) {
B b;
b.m_p->DoSomething();
return 0;
}
#include <iostream>
using namespace std;
class A;
class B {
public:
A* m_p;
B();
void SayHello() { cout << "Hello" << endl; }
~B();
};
class A {
private:
void (B::*m_pFunctionPointer)()
public:
void DoSomething();
void SetFunctionPointer(void (B::*fun_ptr)()) { m_pFunctionPointer = fun_ptr; }
};
void A::DoSomething() {
B b;
(b.*m_pFunctionPointer)();
}
B::B() {
m_p = new A();
m_p->SetFunctionPointer(&B
}
B::~B() {
delete m_p;
}
int main(void) {
B b;
b.m_p->DoSomething();
return 0;
}
Oops, should have checked before posting ... that's the trouble when you start writing a reply, and get distracted half way through :)
Hey DJ,
Try this one: its tidy!
http://www.codeproject.com/cpp/Functor.asp
Best Regards,
DeepuAbrahamK
Try this one: its tidy!
http://www.codeproject.com/cpp/Functor.asp
Best Regards,
DeepuAbrahamK