CODEWITHSUNDEEP
c++functionobjectthisself
Axel Kennedal
Axel Kennedal

Reputation: 555

Use variable from self C++

I'm trying to create a small class that creates "dots" and calculates the distance between them. I first create two "dot"-objects and then pass the second one as an argument to a function contained in the first dot. This gives me access to the coordinates of the second dot, however I don't really get how I'm supposed to get the coordinates of the first dot - the one that receives the second dot as an arg. I figured it would look something like this, but apparently not:

.h

class Punkt{ //"Punkt" = swedish for "dot"

private:
    int x, y;

public:
    //Konstruktor
    Punkt(int firstX, int firstY): x(firstX), y(firstY){};

    //Setters and getters
    int getX(){return x;}

    void setX(int newX){x = newX;}

    int getY(){return y;}

    void setY(int newY){y = newY;}

    //Avstånd till annan punkt
    double distance(Punkt annanPunkt);
};

.cpp

#include "punkt.h"
#include <cmath>

double distance(Punkt annanPunkt){ //"annanpunkt" = "other dot"
    return sqrt(pow((annanPunkt.getX() - self.getX), 2) + 
    pow((annanPunkt.getY() - self.getY, 2))
    }

Upvotes: 1

Views: 6749

Answers (4)

juanchopanza
juanchopanza

Reputation: 227478

First of all, you have declared distance as a member function, so its definition needs to be in the Punkt scope:

double Punkt::distance(Punkt annanPunkt){

Second, you probably don't need a copy of the argument. Pass a const reference:

double Punkt::distance(const Punkt& annanPunkt){

As for accessing your own data members in the function, you can access them by name (x or y):

annanPunkt.getX() - x), 2)

Alternatively, this->x, getX(), this->getX() etc. I find using this everywhere too verbose.

Note that you will need to make your getters const though:

int getX() const {return x;}

or remove them entirely: if you have no invariants to maintain, it may make sense to use public data members.

Note: I would probably make distance a non-member function:

double distance(const Punkt& a, const Punkt& b);

Upvotes: 5

Mike Seymour
Mike Seymour

Reputation: 254631

First, the function definition needs to qualify the name, to indicate that it's defining the member function rather than a separate non-member function:

double Punkt::distance(Punkt annanPunkt)
       ^^^^^^^

Within the function, members can be accessed directly (e.g. getX), or via this (e.g. this->getX). Note that this is a pointer, hence -> rather than .

getX is a function, so you'll need to add () to call it and get the value.

Finally, you need to take care that the parantheses are in the right place, and end the statement with ;

Putting it all together, this should work:

double Punkt::distance(Punkt annanPunkt){
    return sqrt(pow(annanPunkt.getX() - this->getX(), 2) + 
                pow(annanPunkt.getY() - this->getY(), 2));
}

You can remove this->, or even access x and y directly, if you like brevity.

Upvotes: 1

Wojtek Surowka
Wojtek Surowka

Reputation: 21003

C++ does not have self but this, and in methods you can refer to members without any additional syntax, so you can use:

double distance(Punkt annanPunkt){ //"annanpunkt" = "other dot"
    return sqrt(pow((annanPunkt.getX() - getX()), 2) + 
    pow((annanPunkt.getY() - getY(), 2))
}

Upvotes: 0

4pie0
4pie0

Reputation: 29744

In C++ in class member function you have access to all members of a class:

double Punkt::distance( Punkt annanPunkt) {
      // ^ note: distance is member function of Punkt class 
    return sqrt( pow( ( annanPunkt.getX() - x), 2) + 
                                         // ^  this object x coordinate
    pow( ( annanPunkt.getY() - y, 2))
                            // ^  this object y coordinate
}

Upvotes: 0

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