Inner classes

This is a discussion on Inner classes within the C++ Programming forums, part of the General Programming Boards category; Hi All. I just wanted to know that is it possible to have class within a class. If possible....which I ...

  1. #1
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    Inner classes

    Hi All.
    I just wanted to know that is it possible to have class within a class. If possible....which I think is true...how can we access members of inner class.
    I tried to write this code and tried to access member within inner class. But it gave error in statement f.second.b in code below. Can any one help me ...what is correct way to do it.
    Code:
    #include<conio.h>
    #include<stdio.h>
    #include<iostream.h>
    
    class first{
    	public:
    	int a;
    	first(){a=0;}
    	class second{
    			public:
    			second(){b=0;}
    			~second(){}
    			int b;
    
    		      };
    		};
    
    void main(){
    	first f;
    	clrscr();
    	f.a=5;
    	f.second.b=9;
    	cout<<f.a;
    	cout<<"\n"<<f.second.b;
    	getch();
    }

  2. #2
    The superhaterodyne twomers's Avatar
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    Code:
    class outtie
    {
    public:
        class innie
        {
        public:
            int nummie;
        };
    
        // Make an instance of innie 
        innie d;
    };
    
    int main( void )
    {
        outtie o;
    
        o.d.nummie = 42;
    
        return 0;
    }

  3. #3
    carry on JaWiB's Avatar
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    Yes this is possible, but you still need an instance of that inner class to work with. In your case, you could do:
    Code:
    first::second mySecond;
    mySecond.b = 5;

    Another common nested class is an iterator. Here's a short example of something you might see:
    Code:
    class List
    {
    public:
      class iterator
      {
      //...
      };
    
      //common member functions  
      iterator begin();
      iterator end();
    
    };
    
    int main()
    {
       List myList;
       //do some insertions into the list
    
      //declare an iterator and initialize it to the beginnning of the list
       List::iterator it = myList.begin();
    }
    A few more things I should point out (and you'll hear these a lot if you keep reading the forums):

    1. You're using non-standard headers. The appropriate headers are:
    Code:
    #include <iostream>
    #include <cstdio>
    There is no standard equivalent of conio.h, so you might want to consider using another method of pausing the program at the end.

    2. void main is not standard. Use int main. Note that you do not have to explicitly return a value from main (it will implicitly return 0), though it might be considered good practice.

    3. Member data of classes should always be private. You might've just made your variables public for simplicity, but I thought I should note it anyway
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  4. #4
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by JaWiB View Post
    3. Member data of classes should always be private.
    Maybe you should tell that to the designers of STL? For instance, std:: pair

  5. #5
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    >> Maybe you should tell that to the designers of STL? For instance, std:: pair
    pair is a struct, not a class. Although I agree that always is probably the wrong word there.

  6. #6
    carry on JaWiB's Avatar
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    > Although I agree that always is probably the wrong word there.

    I probably should've said class data should almost never be public. I'm sure there are exceptions to the rule, and it's definitely not straightforward to say that data members should never even be protected. I apologize for the poor choice of words on my part, especially since I go by the rule of thumb that whenever someone says always or never, there are usually exceptions.
    "Think not but that I know these things; or think
    I know them not: not therefore am I short
    Of knowing what I ought."
    -John Milton, Paradise Regained (1671)

    "Work hard and it might happen."
    -XSquared

  7. #7
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by Daved View Post
    >> Maybe you should tell that to the designers of STL? For instance, std:: pair
    pair is a struct, not a class. Although I agree that always is probably the wrong word there.
    As far as I know, a struct is just a class with a default access level of public instead of private. So saying "never use public data members" is the same thing as saying "never use structs." Which I think is an odd thing to say...

  8. #8
    C++ Witch laserlight's Avatar
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    As far as I know, a struct is just a class with a default access level of public instead of private. So saying "never use public data members" is the same thing as saying "never use structs." Which I think is an odd thing to say...
    Yes, a struct is indeed a class with default public access. On the other hand, the struct keyword has a C heritage, and in this heritage there is no private access, or member functions for that matter. Conceptually, I think JaWiB was just thinking of the encapsulation that can be enforced by private access, and in that sense it is moving a little further away from C++'s C legacy.
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  9. #9
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    Common usage of structs and classes in C++ is to use structs when you provide public access to the member data, and when using classes encapsulate that data in the private access space. This is not enforced by the language but is still common practice.

    So, with that in mind, "Member data of classes should always be private," is much different than, "Member data of classes or structs should always be private." I was just pointing out that pair is a bad example because it doesn't apply to the original statement.

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