Object Composition
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Thread: Object Composition

  1. #1
    Join Date
    Oct 2013
    Posts
    12

    Object Composition

    Hi I am learning C++, and trying for object composition.

    I have 2 classes: Point and Creature.
    Point->int x and int y
    Creature:-> Point location and string name

    Below is my Point class header

    Code:
    #ifndef POINT_H
    #define POINT_H
    
    class Point
    {
        public:
            /** Default constructor */
            Point(int x, int y);
    
            Point();
    
            void set_x(int);
            void set_y(int);
            int get_x();
            int get_y();
    
        protected:
    
        private:
        int x;
        int y;
    };
    
    #endif // POINT_H
    Below is my Point class cpp
    Code:
    #include<iostream>
    #include "Point.h"
    
    Point::Point(int t, int h)
    {
        x=t;
        y=h;
        //ctor
    }
    void Point::set_x(int h){
        x=h;
    }
    
    void Point::set_y(int h){
        y=h;
    }
    
    int Point::get_x(){
        return x;
    }
    
    int Point::get_y(){
        return y;
    }
    My creature header file looks like:
    Code:
    #ifndef CREATURE_H
    #define CREATURE_H
    #include "Point.h"
    #include <string>
    
    class Creature
    {
        public:
            /** Default constructor */
            Creature(Point &location, std::string name);
            /** Access loc
             * \return The current value of loc
             */
            Point Getloc() { return loc; }
            /** Set loc
             * \param val New value to set
             */
            void Setloc(Point &val) { loc = val; }
            /** Access name
             * \return The current value of name
             */
            std::string Getname() { return name; }
            /** Set name
             * \param val New value to set
             */
            void Setname(std::string val) { name = val; }
        protected:
        private:
            Point loc; //!< Member variable "loc"
            std::string name; //!< Member variable "name"
    };
    
    #endif // CREATURE_H
    My creature cpp looks like this:
    Code:
    #include "Creature.h"
    #include "Point.h"
    
    Creature::Creature(Point &location, std::string n_name)
    {
        //ctor
    
        name=n_name;
        loc=location;
    
    }
    I need to have a creature object with its location. The location will be passed as an object of Point.
    I am getting error
    undefined reference to 'Point::Point()'
    at line:
    Code:
    Creature::Creature(Point &location, std::string n_name)
    I am sure there is some mistake in passing reference, which I cannot figure it out.

  2. #2
    Join Date
    Jul 2005
    Location
    Netherlands
    Posts
    1,999

    Re: Object Composition

    Quote Originally Posted by deathmetal View Post
    Hi I am learning C++, and trying for object composition.
    So why did you not post in the C++ forum?
    Code:
    class Point
    {
        public:
            /** Default constructor */
            Point(int x, int y);
    
            Point();
    ...
    The first is not the default constructor. The default constructor is a constructor that can be called without arguments. So, it either has no arguments or all argument must have a default value. The second constructor above is a default constructor of Point.

    In the constructor of your Creature class, you create an instance of Point (the member loc) without passing any arguments to the constructor. Hence, the default constructor is called, only you did not implement it. You only declared it in your class definition. That's why you get the linker error. To solve it, either implement the default c'tor of Point or use the initializer list in the Creature c'tor to initialize loc with values for x and y (and remove the declaration of the default c'tor of Point).
    Cheers, D Drmmr

    Please put [code][/code] tags around your code to preserve indentation and make it more readable.

    As long as man ascribes to himself what is merely a posibility, he will not work for the attainment of it. - P. D. Ouspensky

  3. #3
    Join Date
    Oct 2013
    Posts
    12

    Re: Object Composition

    Apologies for the inconvenience caused.
    Quote Originally Posted by D_Drmmr View Post
    So why did you not post in the C++ forum?

    Thank you for the help.
    I have now added default constructor.

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