August 14, 2012
On 8/14/2012 3:31 AM, Mehrdad wrote:
> Then you get the best of both worlds:
>
> 1. You force the programmer to manually initialize the variable in most cases,
> forcing him to think about the default value. It's almost no trouble for
>
> 2. In the cases where it's not possible, the language helps the programmer catch
> bugs.
>
>
> Why the heck D avoids #1, I have no idea.

As I've explained before, user defined types have "default constructors". If builtin types do not, then you've got a barrier to writing generic code.

Default initialization also applies to static arrays, tuples, structs and dynamic allocation. It seems a large inconsistency to complain about them only for local variables of basic types, and not for any aggregate type or user defined type.


> It's one of the _major_ features of C# and Java that help promote correctness,
> and #1 looks orthogonal to #2 to me.

I know Java doesn't have default construction - does C#?

As for the 'rarity' of the error I mentioned, yes, it is unusual. The trouble is when it creeps unexpectedly into otherwise working code that has been working for a long time.
August 14, 2012
On Tuesday, 14 August 2012 at 21:13:01 UTC, Walter Bright wrote:
> On 8/14/2012 3:31 AM, Mehrdad wrote:
>> Then you get the best of both worlds:
>>
>> 1. You force the programmer to manually initialize the variable in most cases,
>> forcing him to think about the default value. It's almost no trouble for
>>
>> 2. In the cases where it's not possible, the language helps the programmer catch
>> bugs.
>>
>>
>> Why the heck D avoids #1, I have no idea.
>
> As I've explained before, user defined types have "default constructors". If builtin types do not, then you've got a barrier to writing generic code.

Just because they _have_ a default constructor doesn't mean the compiler should implicitly _call_ them on your behalf.

C# and Java don't.


>> It's one of the _major_ features of C# and Java that help promote correctness, and #1 looks orthogonal to #2 to me.
>
> I know Java doesn't have default construction - does C#?


Huh? I think you completely misread my post...
I was talking about "definite assignment", i.e. the _lack_ of automatic initialization.


> As for the 'rarity' of the error I mentioned, yes, it is unusual. The trouble is when it creeps unexpectedly into otherwise working code that has been working for a long time.

It's no "trouble" in practice, that's what I'm trying to say. It only looks like "trouble" if you look at it from the C/C++ perspective instead of the C#/Java perspective.
August 14, 2012
On Tuesday, 14 August 2012 at 21:22:14 UTC, Mehrdad wrote:
> C# and Java don't.

Typo, scratch Java, it's N/A for Java.
August 14, 2012
On 8/14/2012 2:22 PM, Mehrdad wrote:
> I was talking about "definite assignment", i.e. the _lack_ of automatic
> initialization.

I know. How does that fit in with default construction?
August 14, 2012
On Tuesday, 14 August 2012 at 21:58:20 UTC, Walter Bright wrote:
> On 8/14/2012 2:22 PM, Mehrdad wrote:
>> I was talking about "definite assignment", i.e. the _lack_ of automatic initialization.
>
> I know. How does that fit in with default construction?

They aren't called unless the user calls them.


	void Bar<T>(T value) { }
	
	void Foo<T>()
		where T : new()   // generic constraint for default constructor
	{
		T uninitialized;
		T initialized = new T();
	
		Bar(initialized);  // error
		Bar(uninitialized);  // OK
	}
	
	void Test() { Foo<int>(); Foo<Object>(); }


D could take a similar approach.
August 14, 2012
On Tuesday, 14 August 2012 at 22:57:26 UTC, Mehrdad wrote:
> 		Bar(initialized);  // error
> 		Bar(uninitialized);  // OK


Er, other way around I mean...
August 15, 2012
On 8/14/2012 3:57 PM, Mehrdad wrote:
>> I know. How does that fit in with default construction?
> They aren't called unless the user calls them.

I guess they aren't really default constructors, then <g>.

So what happens when you allocate an array of them?


> D could take a similar approach.

It could, but default construction is better (!).
August 15, 2012
On Wednesday, 15 August 2012 at 00:32:43 UTC, Walter Bright wrote:
> On 8/14/2012 3:57 PM, Mehrdad wrote:
> I guess they aren't really default constructors, then <g>.

I say potayto, you say potahto...  :P


> So what happens when you allocate an array of them?

For arrays, they're called automatically.


Well, OK, that's a bit of a simplification.

It's what happens from the user perspective, not the compiler's (or runtime's).

Here's the full story.
And please read it carefully, since I'm __not__ saying D should adopt what C# does word for word!

In C#:
- You can define a custom default constructor for classes, but not structs.
- Structs _always_ have a zero-initializing default (no-parameter) constructor.
- Therefore, there is no such thing as "copy construction"; it's bitwise-copied.
- Ctors for _structs_ MUST initialize every field (or call the default ctor)
- Ctors for _classes_ don't have this restriction.
- Since initialization is "Cheap", the runtime _always_ does it, for _security_.
- The above^ is IRRELEVANT to the compiler!
  * It enforces initialization where it can.
  * It explicitly tells the runtime to auto-initialize when it can't.
    -- You can ONLY take the address of a variable in unsafe{} blocks.
    -- This implies you know what you're doing, so it's not a problem.


What D would do _ideally_, IMO:

1. Keep the ability to define default (no-args) and postblit constructors.

2. _Always_ force the programmer to initialize _all_ variables explicitly.
   * No, this is NOT what C++ does.
   * Yes, it is tested & DOES work well in practice. But NOT in the C++ mindset.
   * If the programmer _needs_ vars to be uninitialized, he can say  = void.
   * If the programmer wants NaNs, he can just say = T.init. Bingo.


It should work pretty darn well, if you actually give it a try.

(Don't believe me? Put it behind a compiler switch, and see how many people start using it, and how many of them [don't] complain about it!)


>> D could take a similar approach.
>
> It could, but default construction is better (!).

Well, that's so convincing, I'm left speechless!
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