On Monday, 24 February 2014 at 14:14:43 UTC, Tobias Pankrath
wrote:
On Monday, 24 February 2014 at 13:56:01 UTC, Remo wrote:
Hi,
right now I am truing to figure out how the constructors
behave in D2.
Question 1: why it is not possible to create custom ctor for
struct?
The design of D relies on the fact that every type has a T.init
property that is known to the compiler and used when in C++ the
default ctor would get called. In constructors you can rely on
(this == T.init), for example.
You need to pick one T.init or default constructors and D
picked T.init.
The design of D relies on Andrei opinion. He is indeed convinced
that default constructors are impossible.
However, you can write "default constructor" right now like:
struct S
{
Type t;
this(int)
{
t = whather_is_callable_in_CTFE();
}
}
enum E : S
{
A = S(0)
}
void main()
{
E e;
assert (e.t == whather_is_callable_in_CTFE());
}
Since compiler does this, it can also accept straight syntax and
semantic:
struct S
{
Type t;
this() // proxibited default constructor now
{
t = whather_is_callable_in_CTFE();
}
}
What D really lacks is ability to call function in runtime after
struct instance creation like:
struct S
{
@runtime this() {}
~this(){}
}
lowering to:
S s;
s.__runtime_ctor();
However, since compiler does this all over the place (postblits,
copy constructors, destructors, etc.) There is no conceptual
difference between having:
S s
s.__runtime_ctor();
and
S s;
s.__dtor();
In other words, there is no objective necessity not to have
compile time and runtime default struct constructors since
compiler already heavily does conceptually and technically
similar things.
