build events like anothers languages

[Cambalinho]

class test
{
   private:
       string name="";
   public:
       event void changedname();
       void setname(string value)
       {
            name=value;
       }
       string getname()
       {
            return name;
       }
};

test a, b;

void a::changed()
{
    cout <<"name from variable 'a' was changed" << endl;
}

void b::changed()
{
    cout <<"name from variable 'b' was changed" << endl;
}

i get error about 'a' and 'b' not be a class. but using a template can i validate these code?
my problem is that i don't know how to use objects outside of functions:(
these is polymorphism, but not accepted by C++:(

[o3hC5Di1]

Hi there,

a and b are not classes, they are objects of class test. Think of it this way: Classes are blueprints, like a plan for a house. The blueprint however is not a house in itself, someone needs to actually build the house according to the blueprint. The same with classes:

class house
{
    //definition of what a house looks like
};

house my_house; //create an actual house-object 

Furthermore, to use polymorphism you will need to derive class a and b from class test:

struct test
{
    void changed()
    {
        std::cout << "Base test changed";
    }
};


struct a : public test //inherit from test
{
    void changed() //overloading changed()
    {
        std::cout << "Derived A changed";
    }
};



struct b : public test //inherit from test
{
    void changed() //overloading changed()
    {
        std::cout << "Derived B changed";
    }
};

Also, polymorphism only works with pointers. More information:
http://www.cplusplus.com/doc/tutorial/classes/
http://www.cplusplus.com/doc/tutorial/inheritance/
http://www.cplusplus.com/doc/tutorial/polymorphism/


Hope that helps.

All the best,
NwN

[Cambalinho]

1 question: can i transform 'a' and 'b' in static?

[JLBorges]

> build events like anothers languages

In practice, we would use a library. For instance, Boost.Signals2.
http://www.boost.org/doc/libs/1_54_0/doc/html/signals2.html#idp164786720

If we want a home-grown implementation, a crude outline could be along these lines:

#include <functional>
#include <vector>
#include <iostream>
#include <memory>

template < typename CB > struct event
{
    std::vector< std::function<CB> > call_back ;

    template < typename FN > event<CB>& operator+= ( FN fn )
    {
       call_back.emplace_back(fn) ;
       return *this ;
    }

};

struct test
{
    const std::string& name() const { return name_ ; }

    void name( const std::string& new_name )
    { name_ = new_name ; fire_name_changed() ; }

    event< void( const test&, const std::string& ) > name_changed ;

    template < typename FN, typename... ARGS > void subscribe( FN fn, ARGS&&... args )
    {
        using namespace std::placeholders ;
        name_changed.call_back.emplace_back( std::bind( fn, args..., _1, _2 ) ) ;
    }

    private:

         std::string name_ = "anonymous" ;

         void fire_name_changed()
         { for( const auto& f : name_changed.call_back ) f( *this, name() ) ; }
};

void a_function( const test& sender, const std::string& value )
{
    std::cout << "a_function: -\n    name of object at "
               << std::addressof(sender) << "  changed to: '" << value << "'\n\n" ;
}

void another_fun( const std::string& msg, const test& sender, const std::string& value )
{
    std::cout << "another_fun: -\n    msg: '" << msg
               << "'\n    sender: " << std::addressof(sender)
               << "\n    value: '" << value << "'\n\n" ;
}

int main()
{
    test t ;
    t.name_changed += []( const test& sender, const std::string& value )
                      {
                          std::cout << "lambda: -\n    name of object at "
                                     << std::addressof(sender)
                                     << "  changed to: '" << value << "'\n\n" ;
                      } ;

    t.name_changed += a_function ;

    t.subscribe( another_fun, "**** name of object is changed! ****" ) ;

    struct recvr
    {
        void handle_it( const test& sender, const std::string& value ) const
                      {
                          std::cout << "recvr::handle_it -\n    name of object at "
                                     << std::addressof(sender)
                                     << "  changed to: '" << value << "'\n\n" ;
                      }
    };

    recvr r ;
    t.subscribe( &recvr::handle_it, r ) ;

    t.name( "Cambalinho" ) ; // change the name to test it
}

http://ideone.com/OBmXFf

[o3hC5Di1]

I just wanted to say thanks for that JLBorges, very informative as always.
Good use of C++11 <functional> too :)

NwN

[Cambalinho]

NwN: heres the code updated:

class person
{
private:
    string name;
public:
    void changed();
    person()
    {
        Name.setContainer(this);
    }
    void setname(string value)
    {
        name=value;
        changed();
    }

    string getname()
    {
        return name;
    }
    //property <person,string,&person::setname,&person::getname> Name;
};


class Joana : public person //inherit from test
{
    static void changed() //overloading changed()
    {
        std::cout << "hello ";
    }
};

but the changed() function give me an error:(
can you explain to me these error please?
"C:\Users\Joaquim\Documents\CodeBlocks\testevents\main.cpp|21|undefined reference to `person::changed()'|"

[Cambalinho]

sorry JLBorges, but your code is very complex:(
but thank you
i love be more simple and for my new language is better be more simple

[Cambalinho]

NwN: finally works fine. i mistake something;)
i must use virtual functions for polymorphism:

#include <iostream>
#include "test.h"
#include <string>
#include <sstream>

using namespace std;

class person
{
private:
    string name;
public:
    virtual void changed()
    {
        cout <<"";
    }
    person()
    {
        Name.setContainer(this);//these is from my property header file
    }
    void setname(string value)
    {
        name=value;
        changed();
    }

    string getname()
    {
        return name;
    }
    property <person,string,&person::setname,&person::getname> Name;//these is from my property header file
};

class joana : public person
{
    void changed()
    {
        cout << "joana name changed" << endl;
    }
}joana;

class ana : public person
{
    void changed()
    {
        cout << "ana name changed" << endl;
    }
}ana;

int main()
{

    joana.Name="ana";
    cout <<(string) joana.Name<< endl;
    ana.Name="joana";
    cout <<(string) ana.Name<< endl;
    return 0;
}

strange using an object name with class name... but works;)
i think that i can't be more simple than that.

[Cambalinho]

anotherthing that i want share:
you need a class that don't need any object???

class person2
{
private:
    string name;
public:
    void changed()
    {
        cout <<"hello " << name << endl;
    }
    person2()
    {
        Name.setContainer(this);//these is from my property header file
    }
    void setname(string value)
    {
        name=value;
        changed();
    }

    string getname()
    {
        return name;
    }
    property <person2,string,&person2::setname,&person2::getname> Name;//these is from my property header file
}person2;//see the object name igual to class;) 

it's a nice thing;)

[JLBorges]

> sorry JLBorges, but your code is very complex:(
> i love be more simple and for my new language is better be more simple

Yeah. I looked at the thread title 'events like anothers languages', glanced at a bit of NwN's post and then posted my reply. If I were a bit less hasty, read the complete text of your post, I would have realized that my reply was not what you were looking for. (Though, as it turned out in the end, it wasn't a complete waste - NwN found some value in it.)


> strange using an object name with class name... but works;)

The name of a variable can hide the name of a class (or enum) which is in the same scope.
It can be any variable, not just an object of the same type.

// name of the class is 'person'
class person
{
    // ...
};

// the name of a variable can hide the name of a class (or enum) in the same scope
int person = 234 ; // name of variable 'person' hides the name class person

int main()
{
    person = 6 ; // fine; refers to the variable person; class person is hidden


    // person some_person ; // *** error, 'person' referes to the variable, not the class


    // the elaborated-type-specifier clarifies that it is the class person,
    // (not the variable person which is in the same scope)
    class person some_person ; // fine
}

[Cambalinho]

yah.. very strange... maybe now i can do a better class in my new language and convert it to C++ more easy;)
strange question: if C++ is so powerfull why don't use properties and events directly(with it's own functions)?

[o3hC5Di1]

Hi,

You could reverse the question: C++ is powerful because it doesn't push you to use its built-in events system. It gives you the tools you need to build your own, or allows you to use any number of libraries that you wish.

All the best,
NwN

[Cambalinho]

hi... thanks for correct me;)
that's true. and it's because it have less keywords than another languages;)