A way how to overcome object slicing

Question

Is there a way how to step away , overcome object slicing without using new keyword as paramater to function? I have base object

class Person{
    public:
        Person(string name , int age ){
            this -> name = name;
            this -> age  = age;
        }
        virtual void getInfo(){
            cout << "Person " << name << " " << age;
        }
        void add( string name , Person a){
            Person *one = new Person(a);
            m[ name ] = one;
        }
        void print(){
            for( auto &x: m )
                 x.second -> getInfo()
        }
    protected:
        string name;
        int age;
       map< string , Person*> m;
    };

Derived objects

class Kid : public Person{
public:
    Kid(string name, int age):Person(name,age){};
    virtual void getInfo( ){
        cout << "Kid " << name << " " << age;
    }
};
 class Adult : public Person{
    public:
        Adult(string name, int age):Person(name,age){};
        virtual void getInfo( ){
            cout << "Adult " << name << " " << age;
        }
    };

And trying to invoke it such as

   Person one("John", 25);
   one.add("first",Kid("Mathew",15));
   one.add("second",Adult("Leo",55));
   one.print();

Due to object slicing it prints

"Person..."

I have tried using unique_ptr and ended up with same result. I tried declaring copy function such as

vitual clone() const{ return new Person(*this);}

in base clase

and in derived classes i declared

Kid* clone() const{ return new Kid(*this);}
Adult* clone() const{ return new Adult(*this);}

and edited add function.

    void add( string name , Person a){
        Person *one = a.clone();
        m[ name ] = one;
    }

And i was still strucked down by a object slicind. I have tried various things/approaches/methods and all led to object slicing.

Is there a way how to create such thing ( map that holds derivec classes of base class ) without using new keyword in the argument of function e.g

Person one("bla",5);
one.add("asd", new Kid("one",15))

Thanks for help.

// Current state of the code

class Person{
    public:
        Person(string name , int age ){
            this -> name = name;
            this -> age  = age;
        }
        virtual void getInfo(){
            cout << "Person " << name << " " << age;
        }
        void add( string name , const Person &a){
            Person *one = a.clone();
            m[ name ] = one;
        }
        void print(){
            for( auto &x: m ){
                 x.second -> getInfo();
                 cout << endl;
            }
        }
         virtual Person* clone() const{ return new Person(*this);}

    protected:
        string name;
        int age;
        map< string , Person*> m;
    };

class Kid : public Person{
    public:
        Kid(string name, int age):Person(name,age){};
        virtual void getInfo( ){
            cout << "Kid " << name << " " << age;
        }
        Kid* clone() const{ return new Kid(*this);}
    };
 class Adult : public Person{
    public:
        Adult(string name, int age):Person(name,age){};
        virtual void getInfo( ){
            cout << "Adult " << name << " " << age;
        }
        Adult* clone() const{ return new Adult(*this);}
    };

int main(){
     Person one("John", 25);
   one.add("first",Kid("Mathew",15));
   one.add("second",Adult("Leo",55));
   one.print();
}

what do you mean with "using new keyword as paramater to function" ?? If you want to avoid object slicing simply pass a pointer or reference.... — 463035818_is_not_an_ai, Apr 30 '16 at 18:05
Could you please show an example? I have been trying to make it work for a few hours n and i tried passing both reference and pointer without success — Darlyn, Apr 30 '16 at 18:13

score 2 · Accepted Answer · answered Apr 30 '16 at 18:17

2

Change your add() function to accept its parameter by reference:

void add( string name , const Person &a){
    Person *one = a.clone();
    m[ name ] = one;
}

You are slicing the object when you're passing it by value. That copies and slices it. You need to pass the parameter as a reference, instead.

All your clone() methods are already constant. Good.

answered Apr 30 '16 at 18:17

Sam Varshavchik

114,536
5
94
148

it still get sliced printing only "Person.." – Darlyn Apr 30 '16 at 18:18
You didn't really show how you declared your clone() methods. You only paraphrased it. So, any answer would be a guess, at the best. In any case, this is what a debugger is for. Step through your code, line by line, and see which object's clone() method gets invoked. – Sam Varshavchik Apr 30 '16 at 18:21
I did wrote how i declared the clone() function , there is a whole declaration in my quetion. – Darlyn Apr 30 '16 at 18:21
Only as an additional comment, paraphrased, after you showed your code without it. – Sam Varshavchik Apr 30 '16 at 18:22
I have updated the question for whole code , how it looks like – Darlyn Apr 30 '16 at 18:23
Doesn't appear to be. Besides, you misspelled "virtual", in your alleged code. But, whatever the case may be, like I said: use a debugger. Knowing how to use a debugger is a required skill for anyone who expects to become a skilled C++ developer. Step through your code, inside the add() function, that takes a parameter by reference, step into the clone() call, and see which class's clone() method gets invoked. If you claim you get a base class, start again, and set a breakpoint on the base class's copy constructor, to trap when and why it gets sliced. There's your answer. – Sam Varshavchik Apr 30 '16 at 18:25
@trolkura and please dont edit your question to fix problems you were originally asking for. This invalidates already given answer, makes your question pointless for future readers. – 463035818_is_not_an_ai Apr 30 '16 at 18:29

george_ptr · Answer 2 · 2016-04-30T18:10:30.490

0

change your add() to get the Person argument by *

void add( string name , Person* a ){
    m[ name ] = a;
}

change how you pass the object to the add() function to objects allocated on the free store.

Define your own copy constructor in Person class

edited Apr 30 '16 at 18:10

answered Apr 30 '16 at 18:07

george_ptr

558
6
17

What do you mean by that , isnt clone() function enought? Could you put an example please? – Darlyn Apr 30 '16 at 18:10
An API like `void add(std::unique_ptr)` seems to suggest itself. – Kerrek SB Apr 30 '16 at 18:29

score 0 · Answer 3 · answered Apr 30 '16 at 18:27

If you dont want your objects to get sliced, you simply have to pass by reference or pointer:

struct Foo { 
    virtual void print() const {std::cout << "FOO" <<std::endl;} 
};
struct Bar : Foo { 
    void print() const {std::cout << "BAR" <<std::endl;} 
};

void slicingFunc(Foo foo){ foo.print(); }
void nonSlicingFunc(const Foo& foo) { foo.print(); }


int main() {
    Bar b;
    slicingFunc(b);    // prints FOO
    nonSlicingFunc(b); // prints BAR
    return 0;
}

A way how to overcome object slicing

3 Answers3