Re: Getting this to work similar to self in Python
On 2015-07-23 00:22, nurfz wrote:
I think you got overly complicated answers.
I guess I'm confused as to why the D code isn't acting similar to the
Python code in the sense that you would expect this to reference the
speed property of the current instance and not statically reference
the parent. Am I having these issues because these attributes are being
initialized statically?
No, it's not because they're statically initialized. It's because fields
are not polymorphic.
Would using constructors be the way to go about this? I suppose I'm just
trying to find a way to implement fairly clean and intuitive object
oriented inheritance that isn't crippled by getters/setters, is resolved
at compile time, and doesn't impose any kind of runtime cost other than
what you would assume is associated with fundamental level OOP.
Either you can set the value in the constructor or turn speed in to a
method/property. I think it's easiest to set it in the constructor:
class Airplane : Vehicle
{
this()
{
speed = 100;
}
}
--
/Jacob Carlborg
Re: Getting this to work similar to self in Python
On Wednesday, 22 July 2015 at 22:52:22 UTC, John Colvin wrote:
On Wednesday, 22 July 2015 at 22:22:02 UTC, nurfz wrote:
[...]
Fields of classes are not in any way polymorphic in D (this is
the same as C++ and I think java too). Base class members can
be accessed like so:
class Vehicle {
int speed;
void printSpeed() {
writeln(this.speed);
}
}
class Airplane: Vehicle {
this()
{
Vehicle.speed = 100;
}
}
or if you really want to use the same variable name:
class Airplane: Vehicle {
alias speed = Vehicle.speed;
this()
{
speed = 100;
}
}
You can even automatically do that sort of thing by various
means, the shortest/simplest way I can think of would be:
class Airplane: Vehicle {
private @property Vehicle base() { return this; }
alias base this;
this()
{
speed = 100;
}
}
Ok literally ignore all of that, I'm an idiot.
speed is accessible directly, without Vehicle prefix, both from
inside Airplane and outside. However, if you declare another
variable Airplane.speed, it will shadow* Vehicle.speed, leading
to the behaviour you see. Use a constructor instead of declaring
a new variable with the same name.
*as opposed to override or aliasing
It really shows how little I use classes in D...
Re: Getting this to work similar to self in Python
On Wednesday, July 22, 2015 23:19:35 nurfz via Digitalmars-d-learn wrote: Hmm, is there a specific reason aside from the encapsulation violation? It seems needlessly complicated. If you have someone/something that has direct access to your source code, isn't a getter/setter the least of your concerns? It has nothing to do with people having access to your source code. It's leaking how your class is implemented when you provide access to its member variables, which makes it so that code that uses it is more likely to depend on how it's currently implemented, and it makes it harder to change your code later. Users of your class should not depend on the internal implementantion of that class, since it creates unnecessary dependencies. Search on high cohesion and low coupling, and you'll find stuff talking about how your code is more maintainable and of better quality when internals are kept internal, and dependencies across types are kept to a minimum. And remember that just because a getter/setter or property function happens to currently wrap a member variable, doesn't mean that it will later. For instance, you may need to change your code later so that the value in question is actually calculated rather than stored directly in the class. If the member variable is public, then making that change will break all code that uses that member variable, whereas if it's accessed via getters and setters (or property functions), then all of the code that uses that class will continue to work as it did before, which could save you a fair bit of time and effort, and if your code is part of an API that's publicly available, then you avoid breaking a lot of other people's code. And even if your getter/setter/property function will always wrap a member variable, it could be that you'll need to add code to the function(s) later that does extra checks, or sets another variable, or calls another function in addition to getting the current value of the member variable or setting it to a new value. It may not always simply access the variable. If you've made the member variable public, you'll be forced to break existing code when you need to wrap it in a function later. It's just good practice to make all of your member variable private and not provide direct access to any of them. It'll save you from pain later when you have to change how your class works, even if it's as simple as adding log messages. Even worse in D, invariants get called before and after a public member function is called. They don't get called when you access a public member variable. So, if you have a public member variable in a class with an invariant, you're just shooting yourself in the foot, because you can't guarantee the invariant anymore. It's not getting called when the member variable is accessed, and the code accessing it can do whatever it wants to it, including violate the invariant, and you won't catch it until a public member function gets called later, which could be in completely unrelated code. If you start searching for encapsulation and programming, or encapsulation and OOP, or data encaplusation, or anything like unto it, you will very quickly find a lot of stuff telling you to never make your member variables public. It's commonly considered bad practice in languages like C++, Java, C#, D, etc. because it's a mantainance problem when you do, and it really doesn't play well with OOP, since very few languages that support OOP have polymorphic variables (since to do that, you need wrap their access in function calls internally anyway). Does the @property decorator incur a large runtime cost? @property is only applicable to functions and has no effect on the runtime cost, though if a function is not inlined, then it will cost more than accessing a variable directly. So, if your question is whether wrapping member variable access in a getter/setter or a property function costs more, then that depends on whether the call gets inlined. If the call is inlined, then the cost is the same, and if they're not virtual one line functions are going to be inlined if you compile with -inline (which you would normally use in a release build). And if you're comparing with python, D would win regardless, because the only way that python can have polymorphic variables is by looking them up rather than simply accessing them directly as occurs in a language like D, C++, Java, etc. So, python is essentially always wrapping your variable accesses in getters and setters. - Jonathan M Davis
Re: Getting this to work similar to self in Python
Hmm, is there a specific reason aside from the encapsulation violation? It seems needlessly complicated. If you have someone/something that has direct access to your source code, isn't a getter/setter the least of your concerns? Does the @property decorator incur a large runtime cost?
Re: Getting this to work similar to self in Python
On Wednesday, 22 July 2015 at 22:22:02 UTC, nurfz wrote:
How could I get this D code to work similar to this Python code?
So, here is the D code:
import std.stdio;
class Vehicle {
int speed;
void printSpeed() {
writeln(this.speed);
}
}
class Airplane: Vehicle {
int speed = 100;
}
int main() {
auto v = new Vehicle();
auto a = new Airplane();
v.printSpeed(); // 0
a.printSpeed(); // 0 not 100
writeln(v.speed); // 0
writeln(a.speed); // 100
}
Here is the Python code:
class Vehicle:
speed = 0
def printSpeed(self):
print(self.speed)
class Airplane(Vehicle):
speed = 100
if __name__ == __main__:
v = Vehicle()
a = Airplane()
v.printSpeed() # 0
a.printSpeed() # 100
print(v.speed) # 0
print(a.speed) # 100
I guess I'm confused as to why the D code isn't acting similar
to the Python code in the sense that you would expect this to
reference the speed property of the current instance and not
statically reference the parent. Am I having these issues
because these attributes are being initialized statically?
Would using constructors be the way to go about this? I suppose
I'm just trying to find a way to implement fairly clean and
intuitive object oriented inheritance that isn't crippled by
getters/setters, is resolved at compile time, and doesn't
impose any kind of runtime cost other than what you would
assume is associated with fundamental level OOP.
Sorry for the long winded post, but this has just been
confusing me to no end. Hopefully you guys can help me out! :)
Fields of classes are not in any way polymorphic in D (this is
the same as C++ and I think java too). Base class members can be
accessed like so:
class Vehicle {
int speed;
void printSpeed() {
writeln(this.speed);
}
}
class Airplane: Vehicle {
this()
{
Vehicle.speed = 100;
}
}
or if you really want to use the same variable name:
class Airplane: Vehicle {
alias speed = Vehicle.speed;
this()
{
speed = 100;
}
}
You can even automatically do that sort of thing by various
means, the shortest/simplest way I can think of would be:
class Airplane: Vehicle {
private @property Vehicle base() { return this; }
alias base this;
this()
{
speed = 100;
}
}
Re: Getting this to work similar to self in Python
On Wednesday, July 22, 2015 22:22:00 nurfz via Digitalmars-d-learn wrote:
How could I get this D code to work similar to this Python code?
So, here is the D code:
import std.stdio;
class Vehicle {
int speed;
void printSpeed() {
writeln(this.speed);
}
}
class Airplane: Vehicle {
int speed = 100;
}
int main() {
auto v = new Vehicle();
auto a = new Airplane();
v.printSpeed(); // 0
a.printSpeed(); // 0 not 100
writeln(v.speed); // 0
writeln(a.speed); // 100
}
Here is the Python code:
class Vehicle:
speed = 0
def printSpeed(self):
print(self.speed)
class Airplane(Vehicle):
speed = 100
if __name__ == __main__:
v = Vehicle()
a = Airplane()
v.printSpeed() # 0
a.printSpeed() # 100
print(v.speed) # 0
print(a.speed) # 100
I guess I'm confused as to why the D code isn't acting similar to
the Python code in the sense that you would expect this to
reference the speed property of the current instance and not
statically reference the parent. Am I having these issues
because these attributes are being initialized statically?
Would using constructors be the way to go about this? I suppose
I'm just trying to find a way to implement fairly clean and
intuitive object oriented inheritance that isn't crippled by
getters/setters, is resolved at compile time, and doesn't impose
any kind of runtime cost other than what you would assume is
associated with fundamental level OOP.
Sorry for the long winded post, but this has just been confusing
me to no end. Hopefully you guys can help me out! :)
Variables are not polymorphic. Only public or protected member functions are
polymorphic. If you want to override behavior in a derived class, you have
to use a function. And generally, it's not good practice to either have
member variables be public or to reuse their names in derived classes.
Making them public breaks encapsulation, and reusing names makes it too easy
to confuse which name you're dealing with.
A related question on SO:
http://stackoverflow.com/questions/31436506
- Jonathan M Davis
