C# properties using C++ Templates, is it Possible?

[Brent Ritchie]

Hello all,

I have been using C# in my programming class and I have grown quite fond
of C# properties. Having a method act like a variable that I can control
access to is really something. As well as learning C#, I think that it's way
overdue for me to start learning C++ Templates (I've been learning it for
about 5 years now).

I think that adding this type of functionality would be a good exercise
to help learn template programming. But befor I start, does anyone know if
it's even possible to add the functionality of C# properties to C++ using
templates? If so then that is all I need, but if not, can anyone suggest an
easy assignment to help learning template programming?

[Matthias Kaeppler]

Brent Ritchie wrote:

Hello all,

I have been using C# in my programming class and I have grown quite fond
of C# properties. Having a method act like a variable that I can control
access to is really something. As well as learning C#, I think that it's way
overdue for me to start learning C++ Templates (I've been learning it for
about 5 years now).

I think that adding this type of functionality would be a good exercise
to help learn template programming. But befor I start, does anyone know if
it's even possible to add the functionality of C# properties to C++ using
templates? If so then that is all I need, but if not, can anyone suggest an
easy assignment to help learning template programming?

Hi, this is a very interesting question, and since I very much like the
concept of properties in C# (I generally think it's a very nice
language), I came up with this in C++:

---------- 8< --------------------------

class Foo {
public:
Foo(): X(0), Y(0), Z(0) {}
Property<int, GET> X;
Property<int, SET> Y;
Property<int> Z;
};

int main()
{
Foo f;
int val;

f.X = 5; // error, read only
val = f.X; // OK

f.Y = 5; // OK
val = f.Y; // error, write only

f.Z = 5; // OK
val = f.Z; // OK
}

---------- 8< --------------------------

Below is the code which realizes the Property class. It may still be
dirty in one way or another, it's just what I came up by hacking around:

---------- 8< --------------------------

enum permission { GET, SET, GETSET };

template <typename T, permission perm = GETSET>
class Property {
public:
Property(const T& val): value(val) {}
virtual operator T() { return value; }
protected:
T value;
};

template <typename T>
class Property<T, GET> {
public:
explicit Property(const T& val): value(val) {}
virtual operator T() { return value; }
protected:
T value;
};

template <typename T>
class Property<T, SET> {
public:
Property(const T& val): value(val) {}
protected:
T value;
};

---------- 8< --------------------------

For example, it may be a better idea to realize permissions through
inheritance, I don't know, maybe someone else can come up with something
more pleasing, but at least it already works.

Tell me what you think.

Regards,
Matthias

[Roland Pibinger]

On Mon, 21 Nov 2005 19:41:11 +0100, Matthias Kaeppler <vo**@void.co m>
wrote:

For example, it may be a better idea to realize permissions through
inheritance, I don't know, maybe someone else can come up with something
more pleasing, but at least it already works.

Tell me what you think.

There are some property implementations floating around the internet,
e.g. http://www.open-std.org/jtc1/sc22/wg...2004/n1615.pdf

Best wishes,
Roland Pibinger

[Matthias Kaeppler]

Brent Ritchie wrote:

Ok, I see the ctor and the selfnamed operator. This is fine and it is
very elegant. One question though where is operator= ? I used your
implementation and used operator= but it is not here. How does that work
exactly?
You could also use operator=. I used the copy constructor, which also
works. If it isn't declared 'explcicit', the compiler may use it to
convert from the parameter type to the class' type.

How did you achieve readonly access? Because the ctor is declared
explicit?
That's right. An explicit ctor must not be used for implicit type
conversion, so that's fine.
I think I actually understand this for the most part. Except for how did
I do:

Property<int> i(3);
i == 2;

And not have operator= defined?

You would have to define operator==. My code was just supposed to
outline how one could implement a property, it's nowhere complete.

[Matthias Kaeppler]

Here's another Property implementation.

This time I left the whole template thing out completely. It only uses
the preprocessor, and feels way more like C# properties than anything
using templates:

class Outer {
public:

property (MyProp, int) {
self(MyProp, int) {
// init property
}

get(int) {
// do something
return value;
}

set(int) {
// do something
this->value = value;
}
} myProperty, anotherProperty ;

};

You can then use it like this:

Outer o;
int val;

o.myProperty = val;
val = o.myProperty;

o.anotherProper ty = o.myProperty;
assert (o.myProperty == o.anotherProper ty);

This version uses the assignment operator rather than the copy constructor.

#define property(name,t ype) class name
#define self(name,type) private: type value; \
public: bool operator== (const type &rhs) const \
{ return this->value == rhs; } \
public: name ()
#define get(type) public: operator type ()
#define set(type) public: void operator= (const type &value)

However, complex types shouldn't be stored in a property because copying
will be too expensive. It also can't handle "deep" lookup of pointer
types. To achieve that, you will have to rely on template meta
programming I guess.

Regards,
Matthias

[GB]

Brent Ritchie wrote:

Hello all,

I have been using C# in my programming class and I have grown quite fond
of C# properties. Having a method act like a variable that I can control
access to is really something.

What is the advantage of

window.color = blue;

over

window.color(bl ue);

or even

window.change_c olor(blue);

Why not always use the function notation instead of having two different
notations depending on whether you decide to treat something as a
"property". I just don't understand the motivation for making a
distinction between changing or requesting "properties " and other forms
of object interaction.

Gregg

[Brent Ritchie]

"GB" <[e-mail] alid> wrote in message
news:wsRgf.7083 $0h5.3886@duker ead10...

Brent Ritchie wrote:

Hello all,

I have been using C# in my programming class and I have grown quite
fond of C# properties. Having a method act like a variable that I can
control access to is really something.
What is the advantage of

window.color = blue;

over

window.color(bl ue);

or even

window.change_c olor(blue);

Really, the only difference (to me) is purely cosmetic. Although using
functions is one way of doing it, I would rather use window.color = BLUE
because it is a simple assignment with (probably) very simple error checking
needed.
Why not always use the function notation instead of having two different
notations depending on whether you decide to treat something as a
"property". I just don't understand the motivation for making a
distinction between changing or requesting "properties " and other forms of
object interaction.

The driving force behind this is my lazyness and hatred of Accessor/Mutator
pairs. I Like to keep functions that are useful public, and not pollute the
class namespace with nearly useless get/set functions. This way I could hide
the implementation from other programmers who need not know about why
something works just that it does. Also I think properties enforce
encapsulation better. They give a simple interface to hide as many
implementation details as possible.
Gregg

[Ben Pope]

Brent Ritchie wrote:

"GB" <[e-mail] alid> wrote in message
news:wsRgf.7083 $0h5.3886@duker ead10...

Brent Ritchie wrote:

Hello all,

I have been using C# in my programming class and I have grown quite
fond of C# properties. Having a method act like a variable that I can
control access to is really something.

What is the advantage of

window.color = blue;

over

window.color(bl ue);

or even

window.change_c olor(blue);

Really, the only difference (to me) is purely cosmetic. Although using
functions is one way of doing it, I would rather use window.color = BLUE
because it is a simple assignment with (probably) very simple error checking
needed.

Probably simple error checking, or perhaps not, the amount of error
checking is irrelevant if you're encapsulating, right?

Why not always use the function notation instead of having two different
notations depending on whether you decide to treat something as a
"property". I just don't understand the motivation for making a
distinction between changing or requesting "properties " and other forms of
object interaction.

The driving force behind this is my lazyness and hatred of Accessor/Mutator
pairs. I Like to keep functions that are useful public, and not pollute the
class namespace with nearly useless get/set functions. This way I could hide
the implementation from other programmers who need not know about why
something works just that it does. Also I think properties enforce
encapsulation better. They give a simple interface to hide as many
implementation details as possible.

I have to disagree. If it looks like a public variable, people will
expect it to behave like one... should what looks like a simple
assignment throw an exception?

And if you consistantly use the function notation, then you are
simplifying the interface.

Additionally, providing direct access to the members (or appearing to)
doesn't really hide the implementation details.

Having said that:
class Thing {
int property_;
public:
const int& property() const { return property_; }
int& property() { return property_; }
};

class OtherThing {
int property_;
public:
int property() const { return property_; }
void property(int val) { property_ = val; }
};

int main() {
Thing thing;
thing.property( ) = 4;
int myProperty = thing.property( );

OtherThing otherThing;
otherThing.prop erty(4);
int myOtherProperty = otherThing.prop erty();
}
Whats wrong with those two ways of exposing the properties? It's a
pretty simple interface. You don't have to prepend the functions with
get & set, just take advantage of overloading.

Ben Pope

[Brent Ritchie]

"Ben Pope" <be************ *****@gmail.com > wrote in message
news:11******** *************** *************** ***@fe5.teranew s.com...

Brent Ritchie wrote:

"GB" <[e-mail] alid> wrote in message
news:wsRgf.7083 $0h5.3886@duker ead10...

Brent Ritchie wrote:
Hello all,

I have been using C# in my programming class and I have grown quite
fond of C# properties. Having a method act like a variable that I can
control access to is really something.
What is the advantage of

window.color = blue;

over

window.color(bl ue);

or even

window.change_c olor(blue);

Really, the only difference (to me) is purely cosmetic. Although using
functions is one way of doing it, I would rather use window.color = BLUE
because it is a simple assignment with (probably) very simple error
checking needed.

Probably simple error checking, or perhaps not, the amount of error
checking is irrelevant if you're encapsulating, right?

I have to agree. Encapsulation should automatically hide these
implementation details. What I think properties are useful for is not just
an excuse for public variables. What I think properties are useful for is to
create simple to use members that have functionality that makes sense in the
context of thier object. Say I have a Clock object with three members Hours,
Minutes Seconds. Using encapsulation rules I would write out
Accessor/Mutator pairs for each variable. If I wanted to add some value to
Minutes then I would create another function to do that, same with Hours and
Seconds. In these new functions I would again use the accessor/mutator
pairs. I always found that in this context, I always end up with functions
that do normal variable routines but use longer function names. What I
suggest is use a "property" so that they "Look" like public variables but
"Behave" in whichever context they are in. I would rather read:

Clock clock(0, 0, 0);
clock.Minutes += 80;

cout << clock.Hours << " : " << clock.Minutes << endl;

output would read : 1 : 20

This just makes sense for a Clock class to do, and I dont have to keep
typing something like clock.addMinute s(80); It just seems more intuitive to
me like this.

Why not always use the function notation instead of having two different
notations depending on whether you decide to treat something as a
"property". I just don't understand the motivation for making a
distinction between changing or requesting "properties " and other forms
of object interaction.

The driving force behind this is my lazyness and hatred of
Accessor/Mutator pairs. I Like to keep functions that are useful public,
and not pollute the class namespace with nearly useless get/set
functions. This way I could hide the implementation from other
programmers who need not know about why something works just that it
does. Also I think properties enforce encapsulation better. They give a
simple interface to hide as many implementation details as possible.

I have to disagree. If it looks like a public variable, people will
expect it to behave like one... should what looks like a simple assignment
throw an exception?

Actually, after using java for a while I've grown quite fond of
everything being able to throw an exception. This allows me to create code
that can dynamically fix any mistakes that may crop up and notify me right
away that there is a problem with one of my classes or methods or whatever.
Being ready for exceptions from anywhere is always a good thing in my mind.
And if you consistantly use the function notation, then you are
simplifying the interface.

Simplifying yes, Easier to use, maybe. When creating a class, you have
to watch the length of your member names that you use. If you don't then you
run the risk of losing time writing and rewriting long member names
especially on members that add functionality for private variables as they
are usually used most. I think that if it makes sense then symbols are
better.
Additionally, providing direct access to the members (or appearing to)
doesn't really hide the implementation details.

In a way, it does. Take the Clock example again. If I go clock.Seconds++
and clock.Seconds is already equal to 59 then clock.Seconds++ would set
itself to 0 and do clock.Minutes++ at the same time to simulate a real
clock. Someone using this class knows that this happens they just don't have
to know how it actually happens. They are just glad that it does happen with
no effort on their part.
Having said that:
class Thing {
int property_;
public:
const int& property() const { return property_; }
int& property() { return property_; }
};

class OtherThing {
int property_;
public:
int property() const { return property_; }
void property(int val) { property_ = val; }
};

int main() {
Thing thing;
thing.property( ) = 4;
int myProperty = thing.property( );

OtherThing otherThing;
otherThing.prop erty(4);
int myOtherProperty = otherThing.prop erty();
}
Whats wrong with those two ways of exposing the properties? It's a pretty
simple interface. You don't have to prepend the functions with get & set,
just take advantage of overloading.

Nothing is wrong with any of those, they are valid and easy to read. But
the problem is when you add funtionality what happens whan you want to add
one to your property? you would have to right a function say addProperty(int
val) { property += val; } where as Thing.Property+ + or Thing.Property += 1.
is shorter and is still obvious to the user. Also the way you do it, I have
to rewrite the default functionality for each member every time I write a
class. With my template I can have the basic functionality as soon as I
declare it, and extend it as I see fit.
Ben Pope

[Ben Pope]

Brent Ritchie wrote:

"Ben Pope" <be************ *****@gmail.com > wrote in message

And if you consistantly use the function notation, then you are
simplifying the interface.

Simplifying yes, Easier to use, maybe. When creating a class, you
have to watch the length of your member names that you use. If you
don't then you run the risk of losing time writing and rewriting long
member names especially on members that add functionality for private
variables as they are usually used most. I think that if it makes
sense then symbols are better.

OK, I'm not with you:

class Thing {
private:
int minutes_;
int hours_;
public:
int& minutes();
int& hours();
};

Member names for accessors are the same (except the underscore) as the
private member variable.

Additionally, providing direct access to the members (or appearing
to) doesn't really hide the implementation details.

In a way, it does. Take the Clock example again. If I go
clock.Seconds++ and clock.Seconds is already equal to 59 then
clock.Seconds++ would set itself to 0 and do clock.Minutes++ at the
same time to simulate a real clock. Someone using this class knows
that this happens they just don't have to know how it actually
happens. They are just glad that it does happen with no effort on
their part.

Having said that:
class Thing { int property_; public: const int& property() const {
return property_; } int& property() { return property_; } };

int main() { Thing thing; thing.property( ) = 4; int myProperty =
thing.property( ); }
Whats wrong with those two ways of exposing the properties? It's a
pretty simple interface. You don't have to prepend the functions
with get & set, just take advantage of overloading.

Nothing is wrong with any of those, they are valid and easy to read.
But the problem is when you add funtionality what happens whan you
want to add one to your property? you would have to right a function
say addProperty(int val) { property += val; } where as
Thing.Property+ + or Thing.Property += 1. is shorter and is still
obvious to the user.

No you don't; from my trimmed example above:

thing.property( ) += 4;

Perfectly valid code.
Also the way you do it, I have to rewrite the default functionality
for each member every time I write a class. With my template I can
have the basic functionality as soon as I declare it, and extend it
as I see fit.

That's pretty lazy. There shouldn't be that many times you want to
provide direct access to the members, and when you do, you'll want to
minimise access as much as possible, which will require writing a small
amount of code.

Ben Pope