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Subject: Re: [boost] Another implementation of properties.
From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2010-03-03 19:49:30

Mateusz Loskot wrote:
> Zachary Turner wrote:
>> On Mar 3, 2010, at 12:07 PM, "Simonson, Lucanus J"
>> <lucanus.j.simonson_at_[hidden]> wrote:
>>> Usually I think of properties as something attached to an instance of
>>> a class without the need to modify the class.
>>> class Person {
>>> string Name;
>>> Person() : Name() {}
>>> };
>>> Person p;
>>> Property<int, Person> Age(-1); //default age is -1, an error code
>>> Age.set(10, &p);
>>> assert(Age.get(&p) == 10);
>>> Such a thing becomes rather complicated once you try to wire in
>>> removal of property values to the destructor of your object type and
>>> duplication of property values to the copy constructor without the
>>> need to know about all the kinds of properties that might be later
>>> associated with the class at the time the class is defined. Add to
>>> that custom heaps for storing property values and clever schemes to
>>> look them up and you quickly run into a system that is quite
>>> complicated with singletons running around and issues with thread
>>> safety etc etc. I think it is better to keep things simple where
>>> possible and understand clearly what the problem is and why the
>>> problem requires a complicated solution before implementing or using a
>>> complicated solution.
>> I think conceptually the proposed abstraction fits my mental image of
>> "properties" pretty closely, and probably the same for anyone who has
>> made significant use of C#.
>> The main problem they address is providing a level of abstraction over
>> data members allowing you to have logical properties as opposed to
>> physical properties. This allows you to change the implementation
>> details of a class without changing its interface.
> I'm not sure about that. I don't recognize a big difference between
> use of C# properties:
> int age = person.Age;
> and C++ member function:
> int age = person.get_age();

In fact, I should write it this way, to mimic C# syntax closer:

int age = person.age();
person.age() = age;

Best regards,

Mateusz Loskot,
Charter Member of OSGeo,

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