CODEWITHSUNDEEP
c++pointersarduino
MC Emperor
MC Emperor

Reputation: 22977

Object property not changing as expected when using pointers in C++

I'm trying to play some around with classes in C++. Coming from a Java world, they're slightly different.

What I'm trying to do should be obvious: I have a class named SomeClass, which holds a single int. Then I have a class named A, which 

// Create a class holding just an integer.
class SomeClass {
    public:

        int _value;

        SomeClass(int value) {
            this->_value = value;
        }
};

class A {

    private:
        // The property _someClassPointer holds a pointer to a SomeClass object.
        SomeClass *_someClassPointer;
    public:

        // We can set a SomeClass instance as a property of the A class.
        void setSomeClass(SomeClass *someClassPointer) {
            // It copies the local pointer value to the property.
            this->_someClassPointer = someClassPointer;
        }

        // We can retrieve the integer that the SomeClass instance holds.
        // We're assuming _someClassPointer does not point to NULL.
        int getValueOfSomeClass() {
            this->_someClassPointer->_value;
        }
};

void setup() {
    Serial.begin(9600);
}

void loop() {

    // Instantiate SomeClass object with int(5) argument
    SomeClass someClass(5);

    Serial.println(someClass._value); // It prints 5, as expected

    // Instantiate an A object by its default constructor
    A a;

    // Pass the address of someClass to the method
    a.setSomeClass(&someClass);

    // Set the value of someClass to 6
    someClass._value = 6;

    Serial.println(a.getValueOfSomeClass()); // It prints 0
}

How come 0 is printed instead of 6?

Upvotes: 0

Views: 121

Answers (2)

Miguel
Miguel

Reputation: 787

2nd Question: It's the diference between passing arguments by value, by reference and by pointer. Its a little tricky, but you can find a lot of info in the web about this topic.

Upvotes: 0

George Houpis
George Houpis

Reputation: 1729

Your function is not returning a value.

int getValueOfSomeClass() {
    return this->_someClassPointer->_value;
}

EDIT: I should mention that you can remove the 'this->' because it is implied in non-static method.

int getValueOfSomeClass() {
    return _someClassPointer->_value;
}

Upvotes: 6

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