C++ object referencing in classes

Question

I am wondering how to store a reference of an object inside of another object, and also set that reference as a private property. Example (pseudo-code):

class foo
{
    public:
        int size;
        foo( int );
};

foo::foo( int s ) : size( s ) {}

class bar
{
    public:
        bar( foo& );
    private:
        foo fooreference;
};

bar::bar( foo & reference )
{
    fooreference = reference;
}

foo firstclass( 1 );
bar secondclass( firstclass );

As you may be able to see, I just want to be able to store the reference of foo inside this bar class. I know how to simply bring it into a method and use it just in the scope of that method, but here I want to set it as a private property. How would I go about doing this?

score 2 · Accepted Answer · answered Sep 16 '11 at 20:05

2

The same way you define and use any class member.

Make sure you initialise the reference member with the _member-initialiser, instead of just assigning to it after-the-fact in the constructor body; recall that references must be initialised and cannot later be rebound.

class foo
{
    public:
        int size;
        foo( int );
};

foo::foo( int s ) : size( s ) {}

class bar
{
    public:
        bar(foo&);
    private:
        foo& fooreference;
};

bar::bar(foo& reference) : fooreference(reference)
{}

foo firstclass(1);
bar secondclass(firstclass);

answered Sep 16 '11 at 20:05

Lightness Races in Orbit

378,754
76
643
1,055

does it have to be done in the constructor or can I have a separate method for this? – grep Sep 16 '11 at 20:07
@Headspin: Constructor. *All* members are initialised during construction. Always. (That's what construction means!) – Lightness Races in Orbit Sep 16 '11 at 20:08
I guess this becomes a problem for me, as I need to initialize the bar class first, then later on, set the reference of foo. – grep Sep 16 '11 at 20:15
@Headspin: You can't do that. If `fooreference` is a member of your `bar`, then it must be initialised as part of the `bar`'s initialisation. Perhaps consider a `shared_ptr` instead? – Lightness Races in Orbit Sep 16 '11 at 20:17
Gotcha. Thanks for all the info! – grep Sep 16 '11 at 20:32

Tom Kerr · Answer 2 · 2011-09-16T20:39:00.320

1

You cannot reseat a reference, so you have to set it in the initializer list.

struct Foo {};

struct Bar {
  Bar(Foo &foo_) : foo(foo_) {}
  void set(Foo &foo_) { foo = foo_; } // copies, doesn't reseat
  Foo &foo;
};

edited Sep 16 '11 at 20:39

answered Sep 16 '11 at 20:08

Tom Kerr

10,444
2
30
46

Actually, you wouldn't get an error from assigning one `Foo` to another `Foo`; you'd get one from omitting the initialisation, though. – Lightness Races in Orbit Sep 16 '11 at 20:09
@Tomalak Geret'kal Ah yea, my example isn't close to what I meant. It would be a copy/assignment operation – Tom Kerr Sep 16 '11 at 20:29

score 1 · Answer 3 · answered Sep 16 '11 at 20:09

1

bar::bar( foo & reference )
{
    fooreference = reference;
}

fooreference is just another object. By assigning, you are making a copy of the reference. Note that fooreference isn't an alias to the reference.

answered Sep 16 '11 at 20:09

Mahesh

34,573
20
89
115

C++ object referencing in classes

3 Answers3

Linked