C++ equivalent to VB6 Public Property Get/Let

by: evilrixPosted on 2009-11-03 at 06:27:13ID: 25729216

Normal (unmanaged) C++ has no concept of properties. You simulate them by implementing get and set functions.

class myclass
{
public:
void set_someval(int n);
int get someval() const;
};

by: pgnatyukPosted on 2009-11-03 at 06:33:00ID: 25729273

one more way, that is not always acceptable (and, probably, because of it evilrix didn't mention it) is here:

#include <IOSTREAM>
using namespace std;

class CPoint:
{
int m_nX;
int m_nY;
public:
CPoint(int x, int y) : m_nX(x), m_nY(y) {}

int& X() { return m_nX; }
int& Y() { return m_nY; }
};

int main()
{
CPoint point(10, 10);
point.X() = 20;
point.Y() = 40;
cout << " X = " << point.X() << endl;
}

by: richelieu7777Posted on 2009-11-03 at 06:33:25ID: 25729278

Regarding: int get_someval() const;

I know the keyword "const" is treated differently in C than in C++. I think it's indicating that the value that is returned by get_someval() is a constant. But why isn't the declaration instead:

const int get_somveval();

Also, please use get_someval() in a simple calling function so I'm 100% about the use of const.

Then I'll award you the points as I believe you have answered the question.

by: pgnatyukPosted on 2009-11-03 at 06:37:02ID: 25729312

Properties in C++: http://www.codeguru.com/cpp/cpp/algorithms/general/article.php/c13039/

by: evilrixPosted on 2009-11-03 at 06:37:25ID: 25729316

>> I think it's indicating that the value that is returned by get_someval() i
No, it's telling the compiler that this function can be called even if the class myclass is constant because it doesn't modify the inners of the class. It makes the code "const correct"

http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.10

>> const int get_somveval();
That does return a const value but since you are returning by value and not reference or pointer there is nothing to be gained except making life a little harder for the caller.

by: evilrixPosted on 2009-11-03 at 06:40:07ID: 25729339

>> one more way, that is not always acceptable
Yeah, I don't like that because it returns references to the inners of the class and breaks abstraction. Basically, you can modify internal values without any validation, unlike using a setter.

by: evilrixPosted on 2009-11-03 at 06:40:39ID: 25729343

>> I don't like that because it returns references to the inners of the class
Also, returning references to intrinsics can prevent some compiler optimisation :)

by: richelieu7777Posted on 2009-11-03 at 06:41:04ID: 31649421

Thanks for fast response

by: pgnatyukPosted on 2009-11-03 at 06:50:07ID: 25729445

Yes, you are right.
This comment could be a small assist only.

But I don't like public variables even more. Even in a nested class - that's the place where I use it.

by: evilrixPosted on 2009-11-06 at 10:12:35ID: 25761492

I decided to try and come up with a better solution for this... something generic. Well, I think I am about 2/3 the way there. All you need do is subclass Property<T> and implement (as required) your own get and put methods to perform validation and create an instance of it in the parent class. These are then called by a cast operator and assignment operator respectively to simulate the behaviour of a property type.

The code is only slightly more that having to implement a getter and putter but it provides proper property semantics in a safe way. There is a little bit more boilerplate code that using, for example, .Net properties but I think the solution is quite elegant.

template <typename T>
class Property
{
public:
	typedef T value_type;
	
	Property(value_type vt = value_type()) : vt_(vt) {}
	
	// Unfortunately, due to the rules of C++ this must be overridden
	// in the specialised class and then called explicitly.
	Property<int> & operator = (value_type const & vt)
	{
		put(vt);
		return *this;
	}
	
	operator value_type const & () const
	{
		return get();
	}
	
protected:
	// Override these to add specialised validation
	virtual value_type const & get() const { return vt_; };
	virtual void put(value_type const & vt){ vt_ = vt; }; 
	value_type vt_;
}; 
#include <stdexcept> 
class MyClass
{
public: 
	// A property definition
	class Value_ : public Property<int>
	{
	public:
		// Pass assignment on to the base class to handle
		Property<int> & operator = (value_type const & vt)	{
			return Property<int>::operator=(vt);
		}
		
	private:
		// Implement get() if you want to specialise the behavior
		
		void put(value_type const & vt) // Only allow 0 to 99
		{
			if(vt < 0 || vt > 99) {
				throw std::runtime_error("validation error");
			}
			
			Property<int>::put(vt);
		}
		
	} Value; 
}; 
#include <iostream> 
int main()
{
	MyClass myClass;
	myClass.Value = 99;
	
	int foo = myClass.Value;
	
	std::cout << foo << std::endl; 
	try
	{
		myClass.Value = 100; // Fails validation
	}
	catch(std::exception const & e)
	{
		std::cerr << e.what() << std::endl;
	}
}

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C++ equivalent to VB6 Public Property Get/Let

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