Re: C++ Templates - passing a pointer to the member to that member's base class
On Apr 23, 9:17 pm, jonas.and...@hotmail.com wrote:
It is possible to create some fairly easy to use implementations of
properties. For example, notice how when the class is used the
attributes look just like normal accessors.
Even if you do nothing else I think you should lose the Get and Set
names and use functors.
Nice, but one of the things with properties ? la C# that I really like
is that the accessor functions (that I called Get and Set), is that
(as in my suggested implementation ) they are members of the
implementetion class, rather than of a helper class. In this way, the
private parts of the implementation class does not have to be availble
to the proprty class(es). By doing so, the accessor functions can be
seens as any other member function.
The design I'm talking about has the same property. The values are
stored inside the functor which is stored in the class instances.
Internally to the instance access is exactly the same:
// external
string name = person->lastName();
// internal
string name = lastName();
The property implementation should be completely decoupled from the
class that they're used in. They don't need and they shouldn't have
any knowledge of the context that they're used in. You'll find them
much easier to implement like that too.
Also (maybe a bit over-ambitious ;-), I find
string firstName = person->firstName;
person->firstName = "Fred" ;
more clear than
string firstName = person->firstName();
person->firstName( "Fred" );
but again, thats down to details. Thanx for your post.
The downside with the first is that it exposes the internal
representation. You've just blown the encapsulation of Person.
Consider this example (contrived, but the principle is sound
nonetheless).
// external
person->initials();
person->initials( L"KS" );
// internal
initials();
initials( L"KS" );
Now we want to change this to have the initials as a derived member.
We throw out the property and replace it with these:
string initials() const {
return firstName()[ 0 ] + lastName()[ 0 ];
}
void initials( string i ) {
firstName( i[ 0 ] );
lastName( i[ 1 ] );
}
The syntax for using this is exactly the same:
// external
person->initials();
person->initials( L"KS" );
// internal
initials();
initials( L"KS" );
You can do this because the syntax for the accessors and the syntax
for the "property", the functor implementation, is an exact match and
this match is maintained both inside and outside of Person. You get
away with a recompile and not having edit anything.
With your preferred syntax you can implement initials() as a functor
by implementing assignment and adding a cast operator. What you cannot
do though is to replace the actual storage of the attribute with a
derived one because the syntax doesn't match.
Here is a link that explains my reasoning for what we use accessors
for and why we use them here:
http://www.kirit.com/On%20following%20rules/Encapsulation%20is%20a%20Good%20Thing%E2%84%A2
You can implement the storage so that you encapsulate not just for the
code making use of Person, but also for the code inside the Person
class.
K
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