Struct Constructors

[Adam D. Ruppe via Digitalmars-d-learn]

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();