returning reference

[l2u]

Hello

If I have the following data structure:

class options {
public:
	void set_value(int v) { m_value = v; }
	int get_value() { return m_value; }
private:
	int m_value;
};

class someclass {
public:
	options &get_options() { return m_options; }
private:
	options m_options;
};

What is the right way to return m_options by reference?
Isnt this right:

options &get_options() const { return m_options; }

so that I'm allowed to call get_options().set and get functions?

Where should I put the 'const' in this case?

Thanks for help!

[matsp]

options &get_options() const { return m_options; }

This is correct - you are not modifying the content of your class.


--
Mats

[l2u]

Why do I get an error with this:

error C2440: 'return' : cannot convert from 'const options' to 'options &'

[matsp]

Ah, yes, I suppose if you do return a reference to a member variable, you are allowing the code outside the object to modify the object itself, so although the member function itself isn't doing any changing, it will potentially be modified by the new user of the reference: so either no const at all, or const option &get...() const ... [you can have both].

--
Mats

[laserlight]

You have a choice between a getter/setter pair:

const options& get_options() const { return m_options; }
void set_options(const options& new_options) { m_options = new_options; }

Or to provide:

const options& get_options() const { return m_options; }
options& get_options() { return m_options; }

For the latter choice the non-const version of get_options() will be used unless the object is const.

[l2u]

You have a choice between a getter/setter pair:

const options& get_options() const { return m_options; }
void set_options(const options& new_options) { m_options = new_options; }

Or to provide:

const options& get_options() const { return m_options; }
options& get_options() { return m_options; }

For the latter choice the non-const version of get_options() will be used unless the object is const.

Which options is better? What do you guys prefer?

The first option seems better to me, but I guess in that case I have to overload = operator for options class? Will it compile after I do this?

[brewbuck]

The first option seems better to me, but I guess in that case I have to overload = operator for options class? Will it compile after I do this?

You'd have to do that anyway even in the second example, because presumably the caller is obtaining a non-const reference in order to assign to it, right?

[laserlight]

The first option seems better to me, but I guess in that case I have to overload = operator for options class?

I prefer the first option too, and you do not need to write operator= since the compiler provided version will work correctly. Of course, if the compiler provided version would not work correctly, then you should implement it yourself either way.

[Elysia]

I tend to prefer using set/get these days with a little helper class:

	template<typename Type, typename Class> class CUtilityMember
	{
	private:
		Type Data;
		typedef const Type& (Class::*GetFncPointer)();
		typedef void (Class::*SetFncPointer)(const Type& NewData);
		GetFncPointer m_pGet;
		SetFncPointer m_pSet;
		Class* m_pClass;
	
	public:
		CUtilityMember(Class* pClass, GetFncPointer pGet = NULL, SetFncPointer pSet = NULL);
		CUtilityMember(const Type& NewData, Class* pClass, GetFncPointer pGet = NULL,
			SetFncPointer pSet = NULL);
		CUtilityMember& operator = (const Type& NewData);
		operator Type () const;
	};

	template<typename Type, typename Class> CUtilityMember<Type, Class>::CUtilityMember
		(Class* pClass, GetFncPointer pGet, SetFncPointer pSet): m_pClass(pClass), m_pGet(pGet),
		m_pSet(pSet) { }
	template<typename Type, typename Class> CUtilityMember<Type, Class>::CUtilityMember
		(const Type& NewData, Class* pClass, GetFncPointer pGet, SetFncPointer pSet):
		Data(NewData), m_pClass(pClass), m_pGet(pGet), m_pSet(pSet) { }
	template<typename Type, typename Class> CUtilityMember<Type, Class>&
		CUtilityMember<Type, Class>::operator = (const Type& NewData)
	{
		if (m_pSet)
			(m_pClass->*m_pSet)(NewData);
		else
			Data = NewData;
		return *this;
	}
	template<typename Type, typename Class> CUtilityMember<Type, Class>::operator Type () const
	{
		if (m_pGet)
			return (m_pClass->*m_pGet)();
		else
			return Data;
	}

And to use it:

		CFilter(CString strExt = _T(""), CString strDescription = _T("")): strExt(strExt, this),
			strDescription(strDescription, this) { }
		CUtilityMember<CString, CFilter> strExt;
		CUtilityMember<CString, CFilter> strDescription;

Optionally, if you want to extra things rather than just set/get the value of the class, then you can provide a class function pointer to a get/set function within your own class.
It's not 100% finished yet, but it will allow you to do

Myclass.MyVar = somevalue;
And
sometype somevalue = Myclass.MyVar;

Without sacrificing the encapsulation. You can add more operators as necessary.

[l2u]

I prefer the first option too, and you do not need to write operator= since the compiler provided version will work correctly. Of course, if the compiler provided version would not work correctly, then you should implement it yourself either way.

I still get the error if I define the set function as you said. What am I doing wrong?

[laserlight]

I still get the error if I define the set function as you said. What am I doing wrong?

Post the smallest and simplest program that demonstrates the error, and also post the exact error message.

[l2u]

Post the smallest and simplest program that demonstrates the error, and also post the exact error message.

Sorry, my bad - I forgot to put const in front of get_options() function.

[l2u]

I want to make sure about something..

If I have this design:

class options {
public:
	void set_value(int v) { m_value = v; }
	int get_value() { return m_value; }
private:
	int m_value;
};

class subclass {
public:
	subclass(options &opt) : m_options(opt) { }
private:
	options &m_options;
};

class someclass {
public:
	someclass() : m_subclass(m_options) { }

	const options &get_options() const { return m_options; }
	void set_options(const options& new_options) { m_options = new_options; }
private:
	options m_options;
	subclass m_subclass;
};

Now I do:

someclass test;
test.set_options(some_new_options);

Can this make a problem since I pass a reference of m_options to subclass object?
I think this shouldnt be a problem since it will only reassign the values or am I wrong?

[Elysia]

It won't cause a problem because you're assigning to a non-reference type.
But I still question if this approach is a good one. You're going to have to remake that class for every other class that might use it. And I still feel using = to assign and just the name to get is a better approach (I may be wrong on that).

[l2u]

Why will this give the following error:

class someclass {
public:
	const someobject &get_object(std::string str) const { return m_map[str]; }
	void set_object(std::string str, object obj) { m_map[str] = obj; } 
	
private:
	std::map<std::string, someobject> m_map;
};

error C2678: binary '[' : no operator found which takes a left-hand operand of type 'const someclass::map' (or there is no acceptable conversion)