c++: Not able to understand Message Handlers - c++

Question

Actually I am new to writing handlers but somehow i managed to write this piece of code:
#include<iostream>
using namespace std;
class test
{
public:
typedef void (test::*MsgHandler)(int handle);
test()
{
cout<<"costructor called"<<endl;
}
void Initialize()
{
add_msg_Handler(4,&test::System);
}
void System(int handle)
{
cout<<endl<<"Inside System()"<<endl;
cout<<"handle:"<<handle<<endl;
}
protected:
MsgHandler message[20];
void add_msg_Handler(int idx,MsgHandler handler)
{
cout<<endl<<"Inside add_msg_Handler()"<<endl;
cout<<"handler:"<<handler<<endl;
message[idx]=handler;
cout<<"message:"<<message[idx]<<endl;
}
};
int main()
{
test obj;
obj.Initialize();
return 0;
}
This code is working fine, I get the output as:
costructor called
Inside add_msg_Handler()
handler:1
message:1
But there are several things beyond my scope. If I am right System() should have been called in this line:
add_msg_Handler(4,&test::System);
but this is not happening. I need help on rectifying this.
Second thing is, I am not able to understand why I am getting such output:
handler:1
I mean how does handler got initialized to 1.Can somebody help me in solving this??

Answer 1

&test::System is not a function call, it's a pointer to the member function test::System.
(A call would look like System(0) and wouldn't compile if you used it as the parameter in question.)
If you look at the definition of add_msg_handler:
cout<<endl<<"Inside add_msg_Handler()"<<endl;
cout<<"handler:"<<handler<<endl;
message[idx]=handler;
cout<<"message:"<<message[idx]<<endl;
there's not a single place that calls the function handler.
(A call would look like (this->*handler)(0) or (this->*message[idx])(0).)
So the function isn't called because there's nothing in your code that calls it.
The output is 1 because
handler is a pointer to a member function
there's no overload of << for pointers to member functions
there is an implicit conversion from pointer to member function to bool
there's an overload of << for bool
a non-null pointer is implicitly converted to true
true outputs as 1 by default.