• Mark Blume (9/9) Jun 16 2014 Why exactly isn't a constructor without any parameters is not
• Mark Blume (3/12) Jun 16 2014 I should also say that I checked out the documentation already,
• Dicebot (8/17) Jun 16 2014 http://stackoverflow.com/questions/16648273/why-can-i-not-implement-defa...
• Adam D. Ruppe (25/28) Jun 16 2014 The idea is that declaring a plain struct always has almost zero
• Mark Blume (3/3) Jun 16 2014 Adam, Dicebot, Thank you very much for the fast reply. I think

"Mark Blume" <mblume bb.e> writes:

Why exactly isn't a constructor without any parameters is not 
allowed?
Why does
"Struct()" calls "Struct.opCall()," which means "Struct.init" 
initially,
while
"Struct(params)" calls "Struct.this(params)?"

Does "Struct(params)" also call "Struct.opCall(params)?"

I am new to D and I can't seem to understand this.

"Mark Blume" <mblume bb.e> writes:

On Monday, 16 June 2014 at 22:03:28 UTC, Mark Blume wrote:
 Why exactly isn't a constructor without any parameters is not 
 allowed?
 Why does
 "Struct()" calls "Struct.opCall()," which means "Struct.init" 
 initially,
 while
 "Struct(params)" calls "Struct.this(params)?"

 Does "Struct(params)" also call "Struct.opCall(params)?"

 I am new to D and I can't seem to understand this.

I should also say that I checked out the documentation already,
but there is not an actual explanation there.

"Dicebot" <public dicebot.lv> writes:

On Monday, 16 June 2014 at 22:03:28 UTC, Mark Blume wrote:
 Why exactly isn't a constructor without any parameters is not 
 allowed?
 Why does
 "Struct()" calls "Struct.opCall()," which means "Struct.init" 
 initially,
 while
 "Struct(params)" calls "Struct.this(params)?"

 Does "Struct(params)" also call "Struct.opCall(params)?"

 I am new to D and I can't seem to understand this.



Right now I think this decision was not worth it and at least 
CTFE-only constructors should be allowed. But this is not a 
simple thing to change.

Fun fact : originally in D1 days (long time ago in a galaxy far 
away) constructors were not allowed for structs at all. Good it 
has been changed :)

"Adam D. Ruppe" <destructionator gmail.com> writes:

On Monday, 16 June 2014 at 22:03:28 UTC, Mark Blume wrote:
 Why exactly isn't a constructor without any parameters is not 
 allowed?

The idea is that declaring a plain struct always has almost zero 
cost - it is just static data with no extra code run. A zero-arg 
constructor (aka a default constructor in C++) that is run 
automatically on the declaration breaks that idea.

Since sometimes people want it anyway the compromise is to make a 
static opCall and explictly use it with parenthesis. This was 
introduced before D had any struct constructors at all.

Then struct constructors were added to D later and they override 
the old static opCall method... but they must have at least one 
parameter.


So short answer is the confusion is caused by two historical 
features that basically did the same thing. Nowadays, static 
opCall should mostly be avoided. Use constructors instead. Only 
if you need a zero arg constructor should you consider static 
opCall.

 Does "Struct(params)" also call "Struct.opCall(params)?"

If there's no constructor, i think it can. But like i said above, 
you should avoid this because it is mostly just historical 
baggage. Use constructors.

BTW non-static opCall is a different story, that can be useful 
for defining functor objects. That only works on an instance 
object though:

struct Foo { void opCall() { } }

Foo foo; // doesn't call any code, just plain variable declaration
foo(); // calls foo.opCall();

"Mark Blume" <mblume bb.e> writes:

Adam, Dicebot, Thank you very much for the fast reply. I think 
this should be addressed at the documentation since it has 
ability to cause confusion for newbies like me.