C++ class return pointer

Question

I believe this could be a duplicate just because I don't knwo what exact terms should I use to google for the right thing.

I'm working with Arduino and I've written library for communication between multiple Arduinos and Raspberries.

class Machine {
    public:
        // other functions here
        Mission & create_mission(int num);

    private:
        // other variables here
        Mission _missions[10];
    };

And here is the class implementation:

    Mission & Machine::create_mission(int num) {
        Mission mission = Mission();
        mission.setNumber(num);
        _missions[_missions_count] = mission;
        _missions_count += 1;

        return _missions[_missions_count-1];
    }

As I understand, this:

    mission1 = machine.create_mission(1);

does not point to the same mission in _missions array within the class Machine. As I'm not experienced in c++, the question is - how should I implement the function to return the same Mission that's inside the _missions array?

The full code can be downloaded here (.zip): riddle.lv/code/code.zip

Definition should also return by reference. And currently, you have error as you return reference of temporary object. — Jarod42
– Jarod42, Commented Nov 17, 2015 at 13:27
In declaration, you have: Mission & create_mission(int num) and in definition, you have Mission Machine::create_mission(int num) (missing reference). — Jarod42
– Jarod42, Commented Nov 17, 2015 at 13:31
@Swagger it is the case, you should return the reference to your newly created object in the map instead of a reference to the temporary mission. — Hatted Rooster
– Hatted Rooster, Commented Nov 17, 2015 at 13:33

gmoniava · Accepted Answer · 2015-11-17 13:34:48Z

2

Something like this?

    _missions[_missions_count] = mission;
    _missions_count += 1;

    return _missions[_missions_count-1];
}

Also you need to change function signature to

  Mission& Machine::create_mission(int num)

edited Nov 17, 2015 at 13:34

answered Nov 17, 2015 at 13:23

gmoniava

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4 Comments

Swagger Over a year ago

This won't work. I believe that it is the same as I've written it so far

Swagger Over a year ago

I tested that - it still is not the same object when I do this: mission1 = machine.create_mission(1);

eerorika Over a year ago

@Swagger not your whole code, but a minimal reproducible example, yes, put it in the question.

Swagger Over a year ago

I've added the url to the question :)

Hatted Rooster · Accepted Answer · 2015-11-17 13:32:27Z

1

You can either return by reference:

Declaration:

Mission & create_mission(int num);

Definition:

  Mission& Machine::create_mission(int num) {
        Mission mission = Mission();
        mission.setNumber(num);
        _missions[_missions_count] = mission;
        _missions_count += 1;

        return _missions[_missions_count - 1];
    }

Or by pointer:

Declaration:

Mission* create_mission(int num);

Definition:

Mission* Machine::create_mission(int num) {
            Mission mission = Mission();
            mission.setNumber(num);
            _missions[_missions_count] = mission;
            _missions_count += 1;

            return &_missions[_missions_count - 1];
        }

You'd normally return by reference if you expect the object to be there, which is the case here.

Currently, you're returning the reference of a temporary (mission) which is not allowed. _missions[_missions_count] = mission; Will copy your mission object to the map which will outlive the create_mission scope which is what you want.

answered Nov 17, 2015 at 13:32

Hatted Rooster

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4 Comments

Swagger Over a year ago

When I do this using reference mission1 = machine.create_mission(1); mission1 is not the same object that's inside the class

Hatted Rooster Over a year ago

@Swagger it is when returning the reference from the map rather than the temporary object.

Hatted Rooster Over a year ago

@Arkadiy It doesn't? it returns a reference to the copied map object. Recheck the code.

user3458 Over a year ago

@JameyD Right you are, deleting my comment

kandid · Accepted Answer · 2015-11-17 13:38:01Z

0

Your code creates a new Mission object and assigns it to one in your list. Then this temporary object will be returned. Furthermore the create_mission() method is declared to return a copy of a Missionobject.

You can circumvent this by working with references like this:

 Mission &Machine::create_mission(int num) {
    // create a reference to one of the Missions in the array
    Mission &mission = _missions[_missions_count];
    // work on that Mission
    mission.setNumber(num);
    _missions_count += 1
    // return the reference;
    return mission;
}

answered Nov 17, 2015 at 13:38

kandid

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