1) I would rather see a single templated type, call it property<T> which took other template policy parameters denoting "readonly" etc.
What is an advantage? Do you envision other types of properties?
Property is a single concept with different variations. It is clearer to me to make those different variations template parameters rather than introduce them as different class templates.
Clarity is the matter of taste. And IMO could not be driving force in selecting property<T,readonly> over readonly_property<T>. IMO first version present more complex design (and less convenient, for example in actual code it may look like boost::property<T,boost::readonly> ). For such simple facility primary driving force is usability. And you need more strong points to prefer more complex design.
2) Why "obj.member.value = t" rather than "obj.member = t" ? Surely operator == can be overloaded for a write property.
That's very important part of design to make sure that all write access to the property value goes through .value. This way I could textually see where my property get changed. In most cases I just search for property.value in sources.
It is extra code to remember for your users. The expression x.y = somevalue is much easier to use/remember than x.y.value = somevalue. Why make your users pay for something which for you makes it only easier to find something in your code ?
One of the primary reasons I use property instead of public member is an ability to see exactly where property value is get modified (Would I want x.y = ... syntax, I would just use public member). In multimillion code base (not necessarily written by you) an ability to find all places where property is modified is important. Also in my experience read access always prevail write access. So I don't believe it's such a burden.
3) Why "n.p_name -> size()" rather than n.p_name.size()" ? The variable p_name is not a property of type pointer to std::string but rather a property of type std::string.
Because you couldn't override operator. ;-)
The expression "n.p_name" should create a copy of the property, and applying the size() member function gives one the size of the copy. So n,p_name.size() is the correct syntax and not n.p_name -> size(). In my vision of properties, properties are not variables, but values ( ala Borland or MS ). If one wanted to change the size of the property n.p_name, one must write:
std::string x(n.p_name); x.size(10); n.p_name = x;
and not
n.p_name.size(10); // This merely changes the size of a copy
No. here I completely disagree. property layer should be thin and completely negligible at runtime. I would never agree to property design that require for me to create a string copy to be able to modify the property. Also you would never be able to make an expression n.p_name to create a copy of the property. p_name has type property<...>. You may have implicit cast to proxy that hold copy of the property. But whatever you do you will never be able to make m.p_name.size( 10 ) valid. If I have a permission to modify the property I should be able to call non-const property methods. If not I should be able to call const property methods. I do not know other way (other then operator->) to achieve this.
Unfortunately it's quite frequent to have a need for write access. First example is operator= for the owner class. To be able to implement it you need owner class to have write access to the property. And it does not make property "less readonly". That's why I frequently use BOOST_READONLY_PROPERTY( int, (foo) ) p_value; Another idiom that frequently require exclusive write access for some classes is Builder/Factory patterns. Let's say you read class MyClassBuilder that responsible for constructing objects of that class. In most cases you will be forces to make MyClassBuilder a friend for all properties to allow it to set the value. And again in most cases property is still readonly in it's nature. I am open for alternative design proposition, but usability is major player here. After all it simple facility for simple needs.
I understand the need, but I was really arguing semantics. I think "readonly" properties are truly readonly by everyone once they are created. Another name for properties which can be changed only by certain select objects/functions, rather than "readonly", should be used.
Why? p_name is in it's nature readonly property of class node, but deserializer should be able set a value to it when you read stored structure from file. It does not change readonly-ness of the property IMO.
It's actually is more complex idiom than one I am trying to model. But If you have a concrete proposition we may consider it.
Essentially the idea is that getters and setters could be passed as template parameters in the form of boost::function<etc.> type by the property creator. This would allow any function type of object to server as the property getter or setter by the class property creator. Boost::function is an overall solution to function callback. Why not use it with properties if it can be done ?
Because it clearly overkill for all my needs. And in my practice I almost never had a need for something like that. But it shouldn't stop you from working on policy based smart_property framework that would support validation, custom setter/getter, e.t.c. Be aware though that solution complexity should be justified by advantages over other conventional means. For example, why not just have property with custom type MyCustomType. MyCustomType definition may include all the features you mentioned. And I do not see right away how it may be worse than what you propose. Definitely simpler.
If a property were a single type, it would be easier for a pure C++ reflection method to simply find out all public properties of a class and present them as values which could be set by the end-user programmer at design time. Then when a certain object, represented by the design-time RAD creator, is actually created at run-time, all of the properties are automatically set to the values which had been specified by the end-user-programmer at design time. This is the way C++ Builder's Object Inspector and Microsoft's .NET equivalent, called the Component Designer, actually work. Of course both those implementations are using their own C++ extensions to "property" and "reflection", whereas if a property<T> template mechansim were created and reflection became part of the language in 2008, it could be done purely in C++ with no language extensions.
Of course it is just as viable to look for a family of "property" types in a reflection mechanism as a single type, but I prefer the latter as a simplification mechanism.
I wouldn't say it's simplification. And I do not see that much difference between one class or two.
I think the most important thing regarding my concept of a "property" is that it is a value, not a member variable. As such the value can be read and/or written but attempting to take the address of a property, or access its theoretically underlying member variable, would be forbidden.
I do not see anything wrong to allow get property address if you have a permission to do so. This is C++ not Java. And I remember I had a need to do so once somewhere. Gennadiy.