Is there a way you can create an instance of a class that resides within another class?

Question

Is there a way you can create an instance of a class that resides within another class? For example:

class foo
{
public:
    foo()
    {
        //Constructor stuff here.
    }

    class bar
    {
       bar()
       {
           //Constructor stuff here.
       }
       void action(foo* a)
       {
           //Code that does stuff with a.
       }
    }

    void action(bar* b)
    {
        //Code that does stuff with b.
    }
}

Now I just want to create an instance of bar in my main() like this:

foo* fire;
bar* tinder;

But bar is not declared in this scope. The reason I am using a class within a class is because they both use methods that take the other class as a parameter, but I need an instance of each class in my main(). What can I do?

Missed the semi-colons after class definitions. – Cramer Feb 23 '14 at 23:06 — Cramer, Feb 23 '14 at 23:06

score 2 · Answer 1 · answered Feb 23 '14 at 22:44

Now I just want to create an instance of bar in my main() ...

This is how you would do that:

int main()
{
  foo::bar tinder;
}

bar is declared in the scope of foo. It isn't clear why that is, so unless you have a good reason for it, don't use a nested class. Also note that you were trying to declare pointers to foo and foo::bar, not instances.

score 2 · Accepted Answer · answered Feb 23 '14 at 22:45

2

You can use the scope resolution operator: foo::bar* tinder;. This will give you a pointer to a bar, not a bar object. If you want that, you should do foo::bar tinder.

However, you don't have a good reason for using a nested class. You should just put one before the other and then use a forward declaration. Something like:

class foo; // Forward declares the class foo

class bar
{
   bar()
   {
       //Constructor stuff here.
   }
   void action(foo* a)
   {
       //Code that does stuff with a.
   }
};

class foo
{
public:
    foo()
    {
        //Constructor stuff here.
    }

    void action(bar* b)
    {
        //Code that does stuff with b.
    }
};

answered Feb 23 '14 at 22:45

Joseph Mansfield

108,238
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Awesome. you just taught me something new and useful. – DebrisHauler Feb 23 '14 at 23:20
1

Note that you can't do stuff with a before the definition of foo. Instead, define that member after foo. `class bar {public: bar() {...} inline void action(foo *a);};` then later: `void bar::action(foo *a) {...}` – user253751 Feb 23 '14 at 23:31
That answers what would have been my next question. – DebrisHauler Feb 24 '14 at 00:09

score 1 · Answer 3 · answered Feb 23 '14 at 22:46

1

class bar is declared in the scope of class foo. So you have to write

foo::bar* tinder;

Also you forgot to place semicolons after the definitions of class bar and foo.:)

answered Feb 23 '14 at 22:46

Vlad from Moscow

301,070
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Asdine · Answer 4 · 2014-02-24T10:23:20.100

1

Nested classes are declared inside the scope of another class. So to use them from main you need to tell the compiler where to find that class.

Here is the syntax :

foo::bar *tinder;

foo being the parent scope and bar the nested class.

Hope it helped

edited Feb 24 '14 at 10:23

answered Feb 23 '14 at 22:48

Asdine

2,856
1
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10

score 0 · Answer 5 · edited May 23 '17 at 12:28

0

What you want is a so called "nested class".

You can find everything you want to know about this here: Why would one use nested classes in C++?

e.g:

class List
{
    public:
        List(): head(NULL), tail(NULL) {}
    private:
        class Node
        {
          public:
              int   data;
              Node* next;
              Node* prev;
        };
    private:
        Node*     head;
        Node*     tail;
};

edited May 23 '17 at 12:28

Community

1
1

answered Feb 23 '14 at 23:28

Gio

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I was already using a nested class, and that was a part of my problem. – DebrisHauler Feb 24 '14 at 00:13

Is there a way you can create an instance of a class that resides within another class?

5 Answers5