Some thoughts:
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?
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.
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. ;-)
Finally if you need to provide a write access for readonly property you could use another form of readonly_property template. Unfortunately during language limitations (inability to declare template parameter as friend) you would need to use macro, for example like this:
class foo { public: BOOST_READONLY_PROPERTY( int, (foo)(Friend2) ) p_value; ... };
Now only class foo and Friend2 have a write access to the value of the property p_value;
I personally would eliminate this last if the property were truly readonly. It's equivalent in my mind to declaring a const variable but letting only special objects change it. While such an idiom may have its use, I don't think it is the generally correct way to program. The idea of "const" means unchangeable after it has been set and the idea of a readonly property is the same.
Of course I am being too inflexible, but if you wanted a property which could be changed only by select objects, I would have a property policy which would name it something different than readonly, maybe "onlychangeableby" or something more succinct but more understandable than "readonly" for this type of property policy.
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.
If boosters find this functionality worth reusing I am ready to prepare document page describing it's usage. If we don't find this header eligible for review I would like to ask then for permission to at least put these templates in boost namespace (currently boost::unit_test), for it inconvenience for me to keep it in deep namespace for rest of my current development.
I have thought about properties, in the Borland and MS .NET terms, for awhile as being doable in pure C++. I am glad someone else has tackled it even on as simple a basis as you have done for your own use. I believe a slightly fuller treatment might be eventually beneficial. Here are some further ideas:
1) One could use boost::function to specify the read and write functors which get called whenever read or write access to a property's value was needed.
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.
2) As in the Borland property implementation, a property doesn't necessarily need a read/write function but may "point to" a member variable, usually private, which holds the actual property value.
One of the reason I use property templates is to eliminate need for separate definition of value and accessor.
3) Since the range library has been reviewed, a property could have a template parameter which could be a range for the property.
Range of what?
4) A range may not be enough to specify all valid values for a property, so that when a user of the property attempts to set a value which is not valid, the code could throw an exception, assert, or silently ignore the attempted change, all of which actions should be choosable by the end user as a property policy.
Again validators/interpretators may be useful, but it's more complex idiom and may require much more complex design.
4) Properties themselves do not necessarily have to be actual objects. Because of that, all attempts to get an address from a property should be short-circuited. Only its value matters.
You want be able to get a value of readonly property.
5) If properties are a single type, with various policies, once the capabilities of reflection are added to C++ via the work Mr. Stroustrup is doing, it is possible for C++ properties to be used in RAD environments in much the same way that Borland has done with C++ Builder ( now a dead product unfortunately ) and MS is doing with Managed C++/.NET ( still a very alive product thank to Herb Sutter's and others efforts ). This is one of the main reasons I would like to see the pure C++ idea of property as a single templated type with different template policies determining its usage.
RAD environment? I am not sure I follow what you are saying here. Could you elaborate.
Anyway, please don't let all these ideas stop you from offering what you have done for yourself to Boost. They are only thoughts about a more general property facility.
I proposed quite simple practical tool. It may coexist with more complex/powerful solution. But I would like to see more concrete propositions. Gennadiy.