On Tuesday, 9 October 2012 at 18:29:18 UTC, Jonathan M Davis
wrote:
On Tuesday, October 09, 2012 19:08:35 Zhenya wrote:
On Tuesday, 9 October 2012 at 17:21:47 UTC, Zhenya wrote:
> Hi!
> I'm sorry,maybe this topic already was discussed,but could
> anybody explain me
> why default constructor was disallowed in structs?
And if I have to do some initialization of data members,what is
the way to do it?
It's because of the init property. All types in D are
default-initialized to
their init property, and that property must be known at compile
time (meaning
that it doesn't work very well for it to involve a
constructor). For structs,
the init property is decided by what all of the member
variables are directly
initialized to. So,
struct S
{
int i = 5;
string s = "hello";
}
assert(S.init == S(5, "hello"));
S s;
assert(s == S.init);
This has the unfortunate result that we don't get a default
constructor. The
workaround is to declare a static opCall function.
struct S
{
int i = 5;
string s = "hello";
static S opCall()
{
return S(22, "catch");
}
}
S s;
assert(s == S.init);
S s2 = S();
auto s3 = S();
assert(s2 == S(22, "catch"));
assert(s2 == s3);
It doesn't make it so that the struct is default-initialized to
the result of
static opCall, and it doesn't guarantee that the result of
static opCall is
used when no constructor is called, but if you use S()
explicitly, then it
will be used.
A solid design decision in the language (e.g. requiring that
everything be
default-initialized) can have the unfortunate side effect of
restricting other
choices, and in this case, mucks up default constructors for
structs.
- Jonathan M Davis
Ok.Then,can I do my own .init property,that can be executed in
compile time?
