Information on Non-default member class whose argument is another class

[tonysonney]

Hello All,
I have 2 sets of code. Once compiles and the other doesn't. But I don't understand why. Can somebody please help? I started coding cpp just recently. Please excuse me if I am missing anything obvious.

Code which compiles :

#include <iostream>

class classB
{
	public:
		//ctor
		classB(){}

};

class classC
{
	public:
		classB *mb;

		classC()
        {
			std::cout << "Called default" << std::endl;
        }

		//ctor
	 	classC(classB* x)
	 	{
			std::cout << "Called other" << std::endl;
			mb = x;
	 	}
};

class classA
{
	public:
		classB m_aB;
		classC m_aC;

		//ctor
		classA() : m_aC(&m_aB)
		{}
		
};

int main()
{
	classA cA;
	return 0;
}

Code which does not compile :

#include <iostream>

class classB
{
	public:
		//ctor
		classB(){}

};

class classC
{
	public:
		classB *mb;

		classC()
        {
			std::cout << "Called default" << std::endl;
        }

		//ctor
	 	classC(classB* x)
	 	{
			std::cout << "Called other" << std::endl;
			mb = x;
	 	}
};

class classA
{
	public:
		classB m_aB;
		classC m_aC(&m_aB);

		//ctor
		classA(){}
		
};

int main()
{
	classA cA;
	return 0;
}

The error is

g++ test.cpp -o test
test.cpp:33:15: error: expected identifier before ‘&’ token

I don't understand why is it complaining because the identifier is define in line ~32.

Thanks in advance.
Tony

[MikeyBoy]

That's not how you initialise a data member. You need to use an initialisation list in the constructor:

EDIT: THERE IS AN ERROR IN THE CODE BELOW. PLEASE CHECK MY POST LOWER DOWN IN THE THREAD FOR THE CORRECT CODE.

class classA
{
	public:

		classB m_aB;
		classC m_aC(&m_aB);

		//ctor
		classA() :
		  m_aC(&m_aB)
		{}
		
};

You should do this in your creator for classC as well, rather than assigning a value to mb in the body of the constructor.

Edit: What you originally wrote would be interpreted as the declaration of a method called m_aC, with an argument called m_aB that is a reference. The compiler expects there to be a type before the argument, in a method declaration.

[MiiNiPaa]

to declare argument as a reference you should use following syntax: function_name(type& variable_name)
Variable name is optional in declarations.
You have misplaced & and have used variable name instead of type

[tonysonney]

Hello MikeyBoy,
Thank you for the information. Changing Initialisation on m_aC from
1)
classC m_aC
to
classC m_aC
gives the error

g++ test_compile.cpp -o test
test_compile.cpp:33:15: error: expected identifier before ‘&’ token
test_compile.cpp: In constructor ‘classA::classA()’:
test_compile.cpp:36:14: error: class ‘classA’ does not have any field named ‘m_aC’

2) My intention is to pass pointer to m_aB as argument to m_aC (which can be used to call methods of m_aB). My understanding is when I create an object

classA cA;

The memory is created for the class from the top to bottom. So in the case of

public:
		classB m_aB;
		classC m_aC(&m_aB);

the compiler should already have the memory and variable "m_aB" instantiated and known. Isn't it true?

3)I also thought when a class instance is instantiated irrespective as a member class or as a standalone class, the constructor would be called. So in the following code

#include <iostream>

class classB
{
	public:
		//ctor
		classB(){}

};

class classC
{
	public:
		classB *mb;

		//ctor
	 	classC(classB* x)
	 	{
			std::cout << "Called other" << std::endl;
			mb = x;
	 	}
};

class classA
{
	public:
		classB m_aB;
		classC m_aC;

		//ctor
		classA() : m_aC(&m_aB)
		{}
		
};

int main()
{
	classA cA;
	return 0;
}

I expected the
classC m_aC;
line ~28 to fail because the classC does not have a constructor of type
classC(){}
But it compiles fine.
I am by default a C programmer. So I might be missing something specific to cpp :). Any thoughts?

[tonysonney]

Hi MiiNiPaa,
Thank you for the information. My intention is to pass pointer(not reference) to the class instance m_aB. I thought when I instantiate instance of classA member instance of classB will be created. Isnt that the case? If not when would the memory for classB be allocated?

[MikeyBoy]

Ooops, sorry, in the code I posted there was an error. It should be:

class classA
{
	public:

		classB m_aB;
		classC m_aC; // CANNOT initialise m_aC here.

		//ctor
		classA() :
		  m_aC(&m_aB) // Initialise m_aC in the constructor, using an initialisation list
		{}
		
};

My point is that in line 33 of your original code, you CANNOT initialise m_aC. It's just not legal C++. You have to do it in the constructor - preferably using an initialisation list, like I've shown you.

My apologies for the mistake.

[tonysonney]

Hi MikeyBoy,
No apologies required. Any suggestions are welcome. I am grateful for your help.

I am still a bit confiused, what happens when we do
classC m_aC;? In my second post(first post today) I don't have a default constructor for class C. But it still compiles.
Cheers,

[MikeyBoy]

If you don't have any constructors defined at all for a class, then the compiler will create a default constructor implicitly. This constructor will perform no initialisation, and have an empty body - in other words, it will do nothing.

This is necessary because, otherwise, it would be impossible to instantiate an object of that class.

Edit: See, for example:

http://tinyurl.com/kzuw3af

(Link goes to IBM site).

[tonysonney]

I think I understand it now. A class will only call the default constructor of member classes (how ever it is called), unless the modified constructor is given in the initialisation list of the main class. Am I right?

[MikeyBoy]

Yes exactly. Because the initialisation list is the only way to pass any arguments to the constructor of a base class or of a member object. And if you're not passing any arguments, then it has to be the default constructor.

[tonysonney]

Thank you MikeyBoy. Marking this forum as solved.

Cheers,
Tony

[MikeyBoy]

You're welcome!