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digitalmars.D.learn - Why structs and classes instanciations are made differently ?

reply Houdini <svdbg free.fr> writes:
Hello,

I am a C++ coder, and I am learning D (just reading a book, for 
now).

D is very similar to C++ (and also grabs godd ideas from Python), 
but I have a naive question : why does Walter Bright chose to 
instanciate classes like in Java ? And why is it different for 
structs ?
Jul 24
next sibling parent reply Andrea Fontana <nospam example.com> writes:
On Monday, 24 July 2017 at 15:21:54 UTC, Houdini wrote:
 Hello,

 I am a C++ coder, and I am learning D (just reading a book, for 
 now).

 D is very similar to C++ (and also grabs godd ideas from 
 Python), but I have a naive question : why does Walter Bright 
 chose to instanciate classes like in Java ? And why is it 
 different for structs ?
Maybe this will help you: https://stackoverflow.com/questions/10965577/usage-preference-between-a-struct-and-a-class-in-d-language
Jul 24
parent Houdini <svdbg free.fr> writes:
On Monday, 24 July 2017 at 15:37:51 UTC, Andrea Fontana wrote:

 Maybe this will help you:
 https://stackoverflow.com/questions/10965577/usage-preference-between-a-struct-and-a-class-in-d-language
Thanks for this informative link.
Jul 24
prev sibling next sibling parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 7/24/17 11:21 AM, Houdini wrote:
 Hello,
 
 I am a C++ coder, and I am learning D (just reading a book, for now).
 
 D is very similar to C++ (and also grabs godd ideas from Python), but I 
 have a naive question : why does Walter Bright chose to instanciate 
 classes like in Java ? And why is it different for structs ?
Because types with inheritance generally don't work right if you pass by value (i.e. the slicing problem). structs don't support inheritance or virtual functions, so they can be safely passed by value. -Steve
Jul 24
parent reply Houdini <svdbg free.fr> writes:
On Monday, 24 July 2017 at 15:41:33 UTC, Steven Schveighoffer 
wrote:

 Because types with inheritance generally don't work right if 
 you pass by value (i.e. the slicing problem).

 structs don't support inheritance or virtual functions, so they 
 can be safely passed by value.
But in C++, we pass them by reference also to avoid copies (const &). The potential polymorphic usage is not the only point to consider.
Jul 24
next sibling parent via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
On Mon, Jul 24, 2017 at 03:45:29PM +0000, Houdini via Digitalmars-d-learn wrote:
 But in C++, we pass them by reference also to avoid copies (const &).
Exactly... in C++ you basically always pass by reference, so D made that the default.
Jul 24
prev sibling parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 7/24/17 11:45 AM, Houdini wrote:
 On Monday, 24 July 2017 at 15:41:33 UTC, Steven Schveighoffer wrote:
 
 Because types with inheritance generally don't work right if you pass 
 by value (i.e. the slicing problem).

 structs don't support inheritance or virtual functions, so they can be 
 safely passed by value.
But in C++, we pass them by reference also to avoid copies (const &). The potential polymorphic usage is not the only point to consider.
In C++ class and struct are pretty much interchangeable, so technically, class is a wasted keyword for default visibility. In D, I would use classes for any time I need polymorphism, and use structs otherwise. -Steve
Jul 24
next sibling parent Houdini <svdbg free.fr> writes:
On Monday, 24 July 2017 at 17:42:30 UTC, Steven Schveighoffer 
wrote:

 In D, I would use classes for any time I need polymorphism, and 
 use structs otherwise.
OK, I'll adhere to this method. :) Thanks to all for your answers.
Jul 25
prev sibling parent Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Monday, 24 July 2017 at 17:42:30 UTC, Steven Schveighoffer 
wrote:
 On 7/24/17 11:45 AM, Houdini wrote:
 On Monday, 24 July 2017 at 15:41:33 UTC, Steven Schveighoffer 
 wrote:
 
 Because types with inheritance generally don't work right if 
 you pass by value (i.e. the slicing problem).

 structs don't support inheritance or virtual functions, so 
 they can be safely passed by value.
But in C++, we pass them by reference also to avoid copies (const &). The potential polymorphic usage is not the only point to consider.
In C++ class and struct are pretty much interchangeable, so technically, class is a wasted keyword for default visibility. In D, I would use classes for any time I need polymorphism, and use structs otherwise. -Steve
It has also the nice property that porting code from Java/C# is actually really easy when using classes as it has more or less the same semantic. When porting code from C and C++ it is often better to use structs.
Jul 26
prev sibling parent reply Kagamin <spam here.lot> writes:
On Monday, 24 July 2017 at 15:21:54 UTC, Houdini wrote:
 D is very similar to C++ (and also grabs godd ideas from 
 Python), but I have a naive question : why does Walter Bright 
 chose to instanciate classes like in Java ?
C++ is big, there's always something you don't know about it. Java actually instantiates classes the C++ way: http://en.cppreference.com/w/cpp/language/new
Jul 25
parent reply Houdini <svdbg free.fr> writes:
On Tuesday, 25 July 2017 at 15:15:59 UTC, Kagamin wrote:

 C++ is big, there's always something you don't know about it. 
 Java actually instantiates classes the C++ way: 
 http://en.cppreference.com/w/cpp/language/new
Yes, but it isn't the default way in C++ to do dynamic instanciation. Usually, you do static initialization, except when the situation make taht impossible.
Jul 25
parent reply Kagamin <spam here.lot> writes:
On Tuesday, 25 July 2017 at 15:56:45 UTC, Houdini wrote:
 Yes, but it isn't the default way in C++ to do dynamic 
 instanciation.
https://github.com/isocpp/CppCoreGuidelines this? It's only 2 years old. The new operator predates it by decades.
Jul 25
parent reply Houdini <svdbg free.fr> writes:
On Tuesday, 25 July 2017 at 17:16:00 UTC, Kagamin wrote:
 On Tuesday, 25 July 2017 at 15:56:45 UTC, Houdini wrote:
 Yes, but it isn't the default way in C++ to do dynamic 
 instanciation.
https://github.com/isocpp/CppCoreGuidelines this? It's only 2 years old. The new operator predates it by decades.
I meant : When you need to instantiate a class, you usually do : MyClass a; and not : MyClass* a = new MyClass(); You're in a value model. If you find anything in Cpp Guidelines against that, I am interested.
Jul 26
parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 07/26/2017 02:54 AM, Houdini wrote:
 On Tuesday, 25 July 2017 at 17:16:00 UTC, Kagamin wrote:
 On Tuesday, 25 July 2017 at 15:56:45 UTC, Houdini wrote:
 Yes, but it isn't the default way in C++ to do dynamic instanciation.
https://github.com/isocpp/CppCoreGuidelines this? It's only 2 years old. The new operator predates it by decades.
I meant : When you need to instantiate a class, you usually do : MyClass a; and not : MyClass* a = new MyClass(); You're in a value model.
That's my experience as well. However, stack class objects are rare and to repeat Steven, it comes with the problem of slicing. Only after learning D that I realized there were two kinds of C++ types in my code: value types and reference types, latter of which I've achieved with boost::shared_ptr<C>. So, I think D's separation is the right choice. However, classes are unnecessarily expensive due to that 'monitor' member and carry the mistakes of OOP models adopted by Java, C++, and others. I say this under the influence of open multi-methods[1] and anemic domain models[2]. Ali [1] http://forum.dlang.org/thread/cigbfrgipbokyetskypk forum.dlang.org [2] https://www.meetup.com/D-Lang-Silicon-Valley/events/228027468/
Jul 26