a very simple c++ oop question

Question

im struggling with syntax here: hopefully this question is v simple, im just miising the point.

specifically, if i nest a class within another class, so for instance

class a
{
    a  //the constructor
    {
        b an_instance_of_b // an instance of class b
    }
};

class b
{
    public:
    foo()
    {
        cout << "foo";
    }
};

When i try to access this method within B by doing this:

a an_instance_of_a; //declare an instance of a
an_instance_of_a.an_instance_of_b.foo()

^^ this doesnt seem to work. this is simplified (so might be a typo here somewhere). but i know the classes are being setup fine, its just that i cant access the methods inside them if they are nested. what may i be doing wrong?

many thanks.

Answer 1

I do not see a class nested within another class.

I see class "a" end with a bracket-semicolon, before class "b" is started at all. Hence, no nesting.

EDIT

I've reformatted your code to make clear where the classes begin/end.

You are missing a semi-colon after b an_instance_of_b

The constructor of a is private by default. I think you want a public constructor.

EDIT 2

I think this is the code you want:

class b; // forward declare "b" so that it is valid in class "a"

class a
{
    public:
    a(void)  //Constructor is now public.
    {
        // Empty body.  There is now a member variable
    }

    public:
        b a_member_instance_of_b;
};

class b
{
    // Note: no constructor.
    public:
    foo()
    {
        cout << "foo";
    }
};

int main(void)
{
    a an_instance_of_a;
    an_instance_of_a.a_member_instance_of_b.foo();
    return 0;
}

Answer 2

Try this:

class B; // prototype declaration

class hello {
   hello() { //the constructor
      B an_instance_of_b; // an instance of class b
   }
};

class B {
public:
   void foo() {
      cout << "foo";
   }
};

First, they're not nested in your example. There's also no apparent need to do so, anyway.

Second, there were some function and data declaration errors.

Lastly, you need a prototype of B so that the hello::an_instance_of_b declaration can work as a forward reference. Or, just declare B before hello.

Answer 3

Your an_instance_of_b is not a member of a, but a local variable in the constructor of a (and the constructor declaration is missing the parenthesis).

What will happen here is that when you create an instance of a, it creates and immediately destroys an instance of b, then it leaves the constructor for a and the a instance is created.

Answer 4

This works for me with g++ on Linux:

#include <iostream>

using namespace std;

class B {
public:
        void foo() {
                cout << "foo" << endl;
        }
};

class A {
public:
        B b;
};

int main() {
        A a;
        a.b.foo();
}

I'm thinking b might just be private in your case.

Answer 5

Try posting the exact code that compiles & runs. You can cut out un-necessary functions, but the above code won't even compile so it is very hard to nail down the problem.

However taking a stab at it, an_instance_of_b defaults to protected, doesn't it? Which means you will not be able to access it outside of the class. Also judging by your posted code an_instance_of_b isn't even a class variable, it's a local variable within the constructor. You will need to add a Getter to retrieve the instance. Maybe something like this (I haven't done C++ for a while though):

class ClassA
{
  public:
    hello() { _ClassB = new ClassB(); }
    ClassB GetClassB() { return _ClassB; }
  private:
    ClassB _ClassB;
}

class ClassB
{
  public:
    foo() {}
}

void SomeRandomFunction()
{
  ClassA classA = new ClassA();
  classA.GetClassB().foo();
}