Using a method as a variable in C++ - c++

Question

I'm working on an irc bot as a way to help me learn c++ and I was wondering if it is possible to use a method as a variable like this:
//Irc.h
public:
void *onJoin(char* sender, char* channel);
/////
//Main.cpp
void join(char* sender, char* channel)
{
cout << sender << endl;
cout << channel << endl;
}
int main()
{
Irc irc(stuff);
irc.onJoin = join;
}

Answer 1

Yes, it is possible. These variables are called functions pointers. The can write it like this:
void onJoin( char* sender, char * channel );
int main(void)
{
void (*func)(char *,char *);
func = &onJoin;
func( "sender", "channel" );
}
Alternatively you can use std::function<> for that. The code would be the same except for the first line in main() which is replaced by
std::function<void(char*,char*)> func;
This is a bit more legible in my opinion. If you use this, then don't forget to add
#include <functional>
to the top of your file. Instead of using such variables in a function, you can also use them as member variables of any struct or class.

Answer 2

You need a pointer-to-function:
void* (*OnJoinFn)(char*, char*);
In your Irc class,
class Irc
{
public:
OnJoinFn onJoin;
};
This can be assigned as you are doing above:
int main()
{
Irc irc(stuff);
irc.onJoin = join;
}
But I wonder, if you are just learning C++, do you really need a pointer-to-function? pointers-to-function are certianly legal and valid, but an unusual entity and I would typically expect to use some other mechanism. As a start, I would suggest looking in to abstract base classes:
class IIrc
{
public:
virtual void* OnJoin(const char*, const char*) = 0; // pure virtual
virtual ~IIrc() {}; // Don't forget to implement a virtual destructor in any ABC
};
class MyIrc
:
public IIrc
{
public:
void* OnJoin(const char* sender, const char* channel*)
{
// YOUR CODE HERE
}
};
int main()
{
IIrc* irc = new MyIrc;
irc->OnJoin (...);
}
I've taken the liberty of introducing const correctness in OnJoin.
You should also consider not returning a void*, which bypasses most of C++'s type safety mechanisms, but a pointer to an actual object, or another interface.
Finally, using new (and delete, which is missing here, resulting in a memory leak) is poor practice. Instead, prefer to allocate things on the stack or, if you really need dynamic allocation, use a smart pointer.

Answer 3

Whilst this is possible, I would suggest that you're most likely doing something wrong if you need to do this. The TYPICAL C++ way to do "we need to do this in different ways in different circumstances" is to use inheritance:
in irc.h:
class ircBase
{
public:
...
virtual void onJoin(char *sender, char *channel) = 0;
};
in ircXX.h:
class ircXX: public ircBase
{
public:
...
virtual void onJoin(char *sender, char *channel)
{
cout << sender << endl;
cout << channel << endl;
}
};
in ircYY.h:
class ircYY: public ircBase
{
public:
...
virtual void onJoin(char *sender, char *channel)
{
... do something else ...
}
};
And then you create an object of the right kind for what you need.

Answer 4

What you're looking for is a function pointer:
class Irc
{
public:
void (*on_join)(char*, char*);
// ^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^
};
void join(char*, char*);
int main()
{
Irc irc(stuff);
irc.on_join = join;
}
Alternatively, you can use std::function so that you can pass capturing/non-capturing lambdas:
#include <functional>
class Irc
{
public:
std::function<void (char*, char*)> on_join;
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
};
int main()
{
Irc irc(stuff);
irc.on_join = [] (char* sender, char* channel)
{
std::cout << sender << std::endl;
std::cout << channel << std::endl;
};
}