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digitalmars.D.learn - Warnings/What should I know?

reply "DDD" <dcb854d0bfb1 f98b7c56a69c.anonbox.net> writes:
I'm learning D. I'm curious about surprises I may get. I
Oct 17 2013
next sibling parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
On Friday, 18 October 2013 at 06:13:38 UTC, DDD wrote:

 I'm learning D. I'm curious about surprises I may get. I 



Off the top of my head, I think the biggest one is that D doesn't 
offer "default constructors". Instead, it only has a default 
value initialization.

EG:

S s;
S s = S.init;
S s = S();
S s = S(5);

The first 3 lines are more or less equivalent. They will all 
simply initialize s to S.init, which is a compile time known 
value. S(5) will actually call a constructor.

This can bite you in the ass if you write something like:

struct S
{
     this(int i = 5)
     {}
}
S s = S(); //Does *not* call the constructor with the value 5.



Also, if you are coming from C++, know that D bans internal 
pointers. This is actually a sweet deal, because it means the D 
(unlike C++), is free to move objects around without ever having 
to explicitly duplicate them. It simply "bitcopies" the object, 
and never destroys the original. If you thought C++'s "move" 
semantics where sweet, they are nothing compared to D's straight 
up move.

Finally, D doesn't have "copy constructor". It has something a 
bit sweeter called "postblit". Basically, you first bitcopy the 
object you want to duplicate, and then, a function called 
"postblit" gets called, which does the work that is required 
(should any be required). What's nice about this is that the 
"target object" and "source object" never actually communicate.



Finally, if you are coming from C++, then "slices" will be new. 
It is important to understand what these do and don't do: 
http://dlang.org/d-array-article.html If you make the wrong 
assumptions about them, they *will* backfire on you.
Oct 17 2013
next sibling parent reply "simendsjo" <simendsjo gmail.com> writes:
On Friday, 18 October 2013 at 06:56:49 UTC, monarch_dodra wrote:
(...)

 struct S
 {
     this(int i = 5)
     {}
 }
 S s = S(); //Does *not* call the constructor with the value 5.


I didn't know that. In that case, I think struct ctors with only 
optional parameters should be illegal - how is it possible to 
call it?
Struct ctors without parameters is already illegal due to .init.
Oct 18 2013
parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Friday, October 18, 2013 09:02:49 simendsjo wrote:

 On Friday, 18 October 2013 at 06:56:49 UTC, monarch_dodra wrote:
 (...)
 

 struct S
 {
 
     this(int i = 5)
     {}
 
 }
 S s = S(); //Does *not* call the constructor with the value 5.


 
 I didn't know that. In that case, I think struct ctors with only
 optional parameters should be illegal - how is it possible to
 call it?
 Struct ctors without parameters is already illegal due to .init.


I think that there's a bug report on that somewhere, but I'd have to go 
digging for it.

The only way that S() is anything other than S.init is if you overloaded 
opCall and made it static, in which case, S() becomes a call to that.

- Jonathan M Davis
Oct 18 2013
prev sibling next sibling parent "Maxim Fomin" <maxim maxim-fomin.ru> writes:
On Friday, 18 October 2013 at 06:56:49 UTC, monarch_dodra wrote:

 On Friday, 18 October 2013 at 06:13:38 UTC, DDD wrote:

 I'm learning D. I'm curious about surprises I may get. I 



 Off the top of my head, I think the biggest one is that D 
 doesn't offer "default constructors". Instead, it only has a 
 default value initialization.

 EG:

 S s;
 S s = S.init;
 S s = S();
 S s = S(5);

 The first 3 lines are more or less equivalent.


Not always actually.


 This can bite you in the ass if you write something like:

 struct S
 {
     this(int i = 5)
     {}
 }
 S s = S(); //Does *not* call the constructor with the value 5.


One can bite in the ass even more in case of

struct S {  disable this(); } and using S.init or

void foo () { int i; struct S { int bar() { return i; } } and 
also using S.init


 Finally, D doesn't have "copy constructor". It has something a 
 bit sweeter called "postblit". Basically, you first bitcopy the 
 object you want to duplicate, and then, a function called 
 "postblit" gets called, which does the work that is required 
 (should any be required). What's nice about this is that the 
 "target object" and "source object" never actually communicate.


How much are there threads asking what is wrong with the language 
when there are const/immutable structs and postblit :) ?
Oct 18 2013
prev sibling parent reply "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Fri, Oct 18, 2013 at 08:56:45AM +0200, monarch_dodra wrote:

 On Friday, 18 October 2013 at 06:13:38 UTC, DDD wrote:

I'm learning D. I'm curious about surprises I may get. I typically



 
 Off the top of my head, I think the biggest one is that D doesn't
 offer "default constructors". Instead, it only has a default value
 initialization.


[...]

I thought classes do have default ctors?


T

-- 
The day Microsoft makes something that doesn't suck is probably the day they
start making vacuum cleaners... -- Slashdotter
Oct 18 2013
parent "Adam D. Ruppe" <destructionator gmail.com> writes:
On Friday, 18 October 2013 at 17:09:18 UTC, H. S. Teoh wrote:

 I thought classes do have default ctors?


yeah but you still have to new them.

Class c; // c is null, unlike C++

Class c =new Class(); // cool
Oct 18 2013
prev sibling next sibling parent "simendsjo" <simendsjo gmail.com> writes:
On Friday, 18 October 2013 at 06:13:38 UTC, DDD wrote:

 I'm learning D. I'm curious about surprises I may get. I 



Some stuff from the top of my head. Remember that you're asking 
for gotchas and
surprises rather than the nice features of D :)

* D has built-in support for three different character types. 
UTF-8, 16 and 32.
   The default, char and string, is UTF-8. This means .length 
gives the number
   of bytes, not necessarily the number of symbols.

* Private protection is private to the module, not to the type.
   This means a private class method can be called from anywhere 
in the module.

* Array slices are great, but they require some understanding on 
how the
   underlying runtime handles them. See the array slice tutorial.

* D has compile-time reflection rather than runtime. There are 
some features
   related to runtime reflection, but it's quite minimal. It's 
possible to get
   runtime reflection by using compile-time reflection though.

* Templates in D is actually useful, so use them :)

signature, so
   you cannot overload generic functions. In D, this is not a 
problem.
   You can look at std.typecons to get a glimse of how powerful D 
is (warning:
   it still blows my mind, so it's not for the faint of hart, and 
you should
   probably delay this - most D code is actually very nice and 
readable and not
   at all this complex).

* D doesn't have a preprocessor. You can solve much of the same 
stuff by using
   string mixins, template mixins and debug and version 
statements. Note that
   version statements is quite minimal and doesn't support stuff 
like && or ||
   by design.

* The GC isn't as fast as in other languages (yet?), so if you 
are experiencing
   performance problems in a tight loop, try disabling the GC 
during that loop.

* Not a gotcha, but you should look into D specific features like 
transitive
   const/immutable, pure, nothrow, safe, DbC, unittest, -cov, 
final switch,
   scope guards, templates, foreach etc. These change the way you 
code and is
   idiomatic D, so it's nice to learn them early on.
Oct 18 2013
prev sibling parent "Jesse Phillips" <Jesse.K.Phillips+D gmail.com> writes:
On Friday, 18 October 2013 at 06:13:38 UTC, DDD wrote:

 I'm learning D. I'm curious about surprises I may get. I 



D doesn't have the library count other popular languages have.
And there is a graveyard of projects/libraries which is more
noticeable than more popular languages.

The language implementation isn't flushed out/implemented 100%.

You're probably after specifics, but that is mostly subjective.
Pretty much any difference from a familiar language will be a
surprise.
Oct 18 2013