custom loose coupling from string

assume I've a string

Code:

---

string methodName="xyz";

---

Now I want someObject.methodName() or someObject->methodName() which will actually do someObject->xyz();

Now How this job can be performed with C++.
remember the Objectname is known however the methodName is not (e.g. methodName is specified by a string variable)

EDIT

I am not stuck on someObject.methodName() syntax. this can be performed by any syntax. I ultimately call the xyz() method e.g. the method with the same name as the value ofthe string methodName() somehow.

EDIT

Platform Independent Solutions (If possible) are most Welcome

One way would be to make a map of function pointers and call a method which searches the max for "xyx"... Not pretty. Or else in the calling function use an if-else if-else ladder. You can't map a string to a method name like you're trying to do. There's going to have to be an interpreter function which will translate the string to a method call.

Edit: Something like:

Code:

---

class thing {
private:
  typedef void (thing::*_fptr)();
  std::map<std::string, _fptr> mfp;

public:
  thing() {
    mfp["one"] = &thing::one;
    mfp["two"] = &thing::two;
    mfp["three"] = &thing::three;
  }
  void one(){std::cout<<"one\n";}
  void two(){std::cout<<"two\n";}
  void three(){std::cout<<"three\n";}

  void call(std::string func){
    (this->*(this->mfp[func]))();
  }
};

int main( void ) {
  thing t;
  t.call("one");
  t.call("two");
  t.call("three");
}

---

Note, this solution has absolutely no error checking.

I've some idea thats like the following.
in Constroctor callback (function Pointers) for every method will be given through some macro which will be stored in some internal map.
Like
CONTROL("methodName", methodName)

nd on runtime the methodName will be resolved!!

Is there any leackage in this plan ??

However Is it possible to do Assembly level name resoulution at runtime and call ??

Check the edit of my last post. Add parameters as appropriate.

Instead of doing &thing::one; can we use this::one;

Does it work? I'd imagine not, and I'd imagine no. Though I am by no means omniscient.

I am giving your example a try.

ya your code worked. but two questions??

1. Here you are writting the class name &thing. is there anyway to do the same thing without writting the Class name (thats why I was trying to use this instead)
If I use &this->one instead of &thing::one compiler fires errors saying.

Code:

---

ISO C++ forbids taking the address of a bound member function to form a pointer to member function. Say '&thing::one'

---

2. Can the same be handled if case of polymorphic types ?? e.g.
thing extends from matter Class
so can I use &matter::methodOfThing instead of &thing::methodOfThing.
e.g. method will be bound at runtime ??

1) I believe its because member functions belong to classes, not instances. (All instances share the same function, and pass the hidden this argument to identify themselves.)

2) They should support virtual functions. But if the map is defined to contain pointers-to-methods-of-base, then I don't think you can put pointers-to-methods-of-derived in it.

Code:

---

#include <iostream>
#include <map>

class thing {
private:
  typedef void (thing::*_fptr)();
  std::map<std::string, _fptr> mfp;

public:
  thing() {
    mfp["one"] = &thing::one;
    mfp["two"] = &thing::two;
    mfp["three"] = &thing::three;
  }
  virtual ~thing() {}
  virtual void one(){std::cout<<"one\n";}
  virtual void two(){std::cout<<"two\n";}
  virtual void three(){std::cout<<"three\n";}

  void call(std::string func){
    (this->*(this->mfp[func]))();
  }
};

class thingy: public thing
{
public:
  virtual void one(){std::cout<<"thingy_one\n";}
  virtual void two(){std::cout<<"thingy_two\n";}
  virtual void three(){std::cout<<"thingy_three\n";}
};

int main( void ) {
  thingy t;
  t.call("one");
  t.call("two");
  t.call("three");
  thing u;
  u.call("one");
  u.call("two");
  u.call("three");
}

---

1) Might be able to do so with static-ally declared functions. But I'm unsure of this.
2) A per anon's solution.

Function pointers are fun, eh?

So still now the conclution is you must write the class name e.g. &className::methodName

although it doesn't seem much problem in this case as I can use the parent class's name here.

Thanks a lot for your help.

Sorry to Say But this solution doesn't work properly in case of polymorphic Classes.
Here is my example.
Here I've pasted my source code http://gist.github.com/13478.

as you saw I've added a member function .

Code:

---

void addCallback(const std::string& k, _fptr fn){
  mfp[k] = fn;
}

---

Which is not accepting polymorphic types

Polymorphism means that derived classes override existing methods (do the same things, but differently) and not that they add completely new methods. Also to make classes polymorphic, the methods must be virtual.

You can't add matter::four, because that would mean that somehow you should be able to do thing.call("four"), even though thing doesn't have this method.

Perhaps each derived class could keep its own map, and when a call is made, the derived class searches in both maps?

You might also explain the bigger picture what you are trying to do all this.

What you're trying to do there is make a parent know about a child class. The function pointers are called via thing. Thing's methods can be called via matter. But matter's functions can't be called via thing, without some casting... I really don't know how good an idea this is, but it does the job (I highlighted the key area in red):

Code:

---

// thing.h
class thing;
typedef void (thing::*_thing_fptr)();

class thing {
protected:
  std::map<std::string, _thing_fptr> mfp;

public:
  thing();
  virtual void one();
  virtual void two();
  virtual void three();
  virtual void add( const std::string &f_alias, _thing_fptr fptr );

  virtual void call( const std::string &func);
};

// thing.cpp
thing::thing(){
  mfp["one"] = &thing::one;
  mfp["two"] = &thing::two;
  mfp["three"] = &thing::three;
}
void thing::one(){
  std::cout<<"One called (thing)\n";
}
void thing::two(){
  std::cout<<"Two called (thing)\n";
}
void thing::three(){
  std::cout<<"Three called (thing)\n";
}
void thing::add( const std::string &f_alias, _thing_fptr fp ) {
  mfp[f_alias] = fp;
}

void thing::call( const std::string &func ) {
  (this->*(this->mfp[func]))();
}





// matter.h
class matter;
typedef void (matter::*_matter_fptr)();

class matter : public thing {
private:

public:
  matter(){}
  virtual void one();
  virtual void four();
  virtual void add( const std::string &f_alias, _matter_fptr fp );
  ~matter(){}
};

// matter.cpp
void matter::four() {
  std::cout<<"Four called (matter)\n";
}
void matter::one() {
  std::cout<< "Four called (matter)\n";
}
void matter::add( const std::string &f_alias, _matter_fptr fp ) {
  mfp[f_alias] = reinterpret_cast<_thing_fptr>( fp );
}


// main
int main( void ) {
  // Add basic
  thing t;
  t.call("one");
  t.call("two");
  t.call("three");

  // Add alias
  t.add( "my_one", thing::one );
  t.call( "my_one" );

  // child
  matter m;
  m.add( "fourth_one", matter::four );
  m.call( "fourth_one" );

  return 0;
}

---

Edit: Actually. For those who are more in the know than I is that code dangerous? It works, but it's clearly weird and freaky - a child function is being casted to its parent function pointer typedef and child function doesn't exist in its parent.

Thanks for your example.
I forgot about reinterpret_cast.
I can use

Code:

---

m.addCallback("four", reinterpret_cast<void (thing::*)()>(&matter::four));

---

from the child Class with no further change to it.

But one thing I'd like to know with Signal Slot mechanism qt binds methods to objects at runtime.
and I've also seen that Qt prints an error message at runtime if it cannot bind the method with the specified object (e.g. method not found)
How does this work ??

I would like to echo anon and ask why do you want to do this in the first place.

I'll echo the same query and reiterate my question on the safety of casting a child method pointer to its parent. Is this safe? Seems to be breaking a lot of things I wouldn't have thought possible, to be honest.

The Purpouse is simply Play a puzzle game with C++
Well this is the architecture of my wired application
--------------------
There is a Map like.
buy => theBuyMethod()
sell => theSellMethod()
accept a LISTENERNAME as string.
Now you have to call the method designated to the LISTENER
e.g. If the user enters buy call theBuyMethod.

I cant use a switch case here cause I want it to bind automatically.
If I use a switch case I need to update the switch case each time I add a new member.

am I able to make you understand.

If you already have a map, why not just find the string in the map's keys? If it is found, call the function that the key maps to.

You would need to update the map each time you add a new function, but this is the point of the map.

This is what currently going on.
But its showing problems (which is solved using reinterpret_cast) during polymorphic Classes.

Ah yes, I see that now. It did sound very much like what twomers proposed.

At the moment, one problem is that you have to map to the actual member functions themselves, so polymorphism is problematic. One possible solution that comes to mind is to map to a free function that takes an object by (const) reference and then calls the appropriate member function. Polymorphism will then work as intended. For example:

Code:

---

#include <iostream>
#include <map>
#include <string>

class Base
{
public:
    virtual void foo() const
    {
        std::cout << "Base::foo()" << std::endl;
    }
};

class Derived : public Base
{
public:
    virtual void foo() const
    {
        std::cout << "Derived::foo()" << std::endl;
    }
};

void foo(const Base& x)
{
    x.foo();
}

typedef void (*MappedFunc)(const Base&);

int main()
{
    std::map<std::string,MappedFunc> functions;
    functions["foo"] = foo;

    Derived derived;
    std::map<std::string,MappedFunc>::iterator iter = functions.find("foo");
    if (iter != functions.end())
    {
        iter->second(derived);
    }
    else
    {
        std::cout << "No such function." << std::endl;
    }
}

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