Class using multiple instance of another class

Question

I want to learn how to use class object within class, especially how to pass an arguments between the objects. I'm trying to create an example object Calc which should provide two methods Calc.sum and Calc.sub. It should use another object Plus as a math engine. The Plus.add function compile and works well. But I don't know how to initiate multiple instance of plus in the calc. The code:

class Plus{                     // This class works well
    public:
        Plus(int);              // Structure
        int add(int);           // Public method (function)
        int myVar;              // Public property. Just to hold a value.
    private:
        int _init;              // Class-level private variable
};

Plus::Plus(int init){           // Constructor
    _init = init;
}

int Plus::add(int p){           // Method add
    return _init + p;
}

/***************************************************************************/

class Calc{
    public:
        Calc(int);              // Structure
        int sum(int);           // Method sum
        int sub(int);           // Method sub
        int myVar;              // Public property
    private:
        Plus positive(int);     // Class-level private object definition ?
        Plus negative(int);     // This is probably wrong ??
};

Calc::Calc(int init){           // Constructor (also wrong...)
    Plus positive(init);        // Create object "positive" and pass the initial value
    Plus negative(-init);       // Create object "negative" and pass the initial value
}

int Calc::sum(int n){
    return positive.add(n);
}

int Calc::sub(int n){
    return negative.add(n);
}

/***************************************************************************/

Plus two(2);        // Create class object two
Calc five(5);       // Create class object five

void setup(){
    Serial.begin(115200);

    Serial.print("two.add(3) = ");
    Serial.println(two.add(3));         // Calling instance of class Plus
    two.myVar = 100;
    Serial.println(two.myVar);

    Serial.print("five.sum(3) = ");
    Serial.println(five.sum(3));        // Calling instance of class Calc

    Serial.print("five.sub(3) = ");
    Serial.println(five.sub(3));        // Calling instance of class Calc
}

void loop(){}

My example is inspired by this article: http://arduinoetcetera.blogspot.cz/2011/01/classes-within-classes-initialiser.html but the code there is for one instance only

1) How to declare multiple instances of Plus within Calc

2) Is the terminology (comments) right?

Jean-François Fabre · Accepted Answer · 2016-07-31 19:55:23Z

1

Your problem lies here

Calc::Calc(int init){           // Constructor (also wrong...)
    Plus positive(init);        // Create object "positive" and pass the initial value
    Plus negative(-init);       // Create object "negative" and pass the initial value
}

By doing this, you're creating local variables with the same name as your members: it has no effect.

Should be

Calc::Calc(int init): positive(init),negative(-init)
{
}

That way you initialize your members even if your members don't have a default constructor.

Oh and that is also wrong in definition of Calc

    Plus positive(int);     // Class-level private object definition ?
    Plus negative(int);     // This is probably wrong ??

replace by

    Plus positive;     // Class-level private object definition
    Plus negative;

edited Jul 31, 2016 at 19:55

answered Jul 31, 2016 at 19:03

Jean-François Fabre♦

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2 Comments

Combinatix Over a year ago

It does not compile. There must be something else wrong. Sorry, I've spend whole day googling for sollution and I may be tired.

Jean-François Fabre Over a year ago

I found another error by "manual compilation by hand" :), See my edit. Next time please post compilation errors. "it does not compile" is just not enough.

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