C++ Returning a 2D array of objects from a function to use in another function

Question

I've been working on a C++ project for school and I've encountered some problems when it comes to 2D arrays of objects. So I have the following class:

class mecanice
{
public:

    mecanice();
    ~mecanice();
    ...
protected:

private:
    ...
    MyClass Ob[8][8];
};

And I need a method that will return Ob so that I can use it in a method from some other class that will do stuff based on what's in Ob:

class doodle
{
public:
    doodle();
    ~doodle();
    void do_stuff(MyClass M[8][8]);
    ...
protected:

private:
    ...
};

I've tried all kinds ways of going about it but all of them ended up with compiling errors usually related to pointers. Is there any way to do this?

Have you tried to use typedef MyClass MyClassArray[8][8]; and returning Ob as a MyClassArray? — Franck
– Franck, Commented Nov 19, 2016 at 23:02

Vlad from Moscow · Accepted Answer · 2016-11-19 23:29:34Z

3

Try the following

const MyClass ( * myMagicMethod() )[8] const;

Or

typedef const MyClass ( *A )[8];

//...

A myMagicMethod() const;

The qualifier const is used if you do not want that the array would be changed in a method of other class.

Another approach is to pass the array by reference. For example

const MyClass ( & myMagicMethod() )[8][8] const;

Or

typedef MyClass ( &A )[8][8];

//...

const A myMagicMethod() const;

In the method that will process the array the parameter also must be declared like reference.

edited Nov 19, 2016 at 23:29

answered Nov 19, 2016 at 23:07

Vlad from Moscow

313k27 gold badges204 silver badges358 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Justin Time - Reinstate Monica Over a year ago

Personally, I would use typedef MyClass (A)[8][8]; (or typedef decltype(Ob) A;), and have the function return const A&, to clarify that the function is returning a reference (and so the typedef can also be used to declare local variables, if necessary). That's just personal preference, though.

FullStackOverflowDev Over a year ago

Thank you very much, I managed to get it to work now by just passing the array by reference. I didn't know about the sytanx (&MyMethod())[8][8] which is why I was getting all kinds of errors. Side question: why is there another const at the end? I get a compiler error if I leave it there (using CodeBlocks with MinGW that comes with it btw).

Vlad from Moscow Over a year ago

@GeorgeTodosi As I wrote in my answer you can simplify the syntax by using typedef.:)

BizarreCake · Accepted Answer · 2016-11-19 23:07:11Z

0

You cannot return arrays from functions. In your case, a function that looks like:

MyClass[][] bar ();

is ill-formed.

You can consider to have that function take MyClass[][] as a parameter, instead of returning it as a return value, and filling it appropriately.

void bar (MyClass foo[8][8]);

You can also return a pointer instead (which is legal):

MyClass** bar () { }

But you will have to make sure that the memory of the object you are returning a pointer to is persistent (i.e. do not return a pointer to a local variable, etc...).

answered Nov 19, 2016 at 23:07

BizarreCake

6925 silver badges15 bronze badges

Collectives™ on Stack Overflow

C++ Returning a 2D array of objects from a function to use in another function

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related