• Sean Eskapp (3/3) Feb 12 2011 With the removal of scope classes, the only way of which I know to use R...
• Jonathan M Davis (34/37) Feb 12 2011 It's because they have to have an init property. _All_ types have an ini...
• Sean Eskapp (2/39) Feb 12 2011 Alright, thanks for clearing that up!

Sean Eskapp <eatingstaples gmail.com> writes:

With the removal of scope classes, the only way of which I know to use RAII
objects is to use structs. This is fine, but structs aren't allowed default
constructors! Why is this?

Jonathan M Davis <jmdavisProg gmx.com> writes:

On Saturday 12 February 2011 10:48:04 Sean Eskapp wrote:
 With the removal of scope classes, the only way of which I know to use RAII
 objects is to use structs. This is fine, but structs aren't allowed default
 constructors! Why is this?

It's because they have to have an init property. _All_ types have an init 
property. It's what they're default initialized to. For bool, it's false. For 
integral types, it's 0. For floating point types, it's NAN. Etc. For classes, 
that's null, so they can have default constructors just fine. However, for 
structs, it must be what the struct's member variables are directly initialized 
to. If you could have an arbitrary default constructor, then you'd have the 
problem of it throwing exceptions as well as it trying to run functions which 
could only be run at runtime. The struct's init value must be known at compile 
time, and it must _always_ be the same. So, it can't involve any functions
which 
would have to be run at runtime, and it can't involve anything that would
result 
in differing values betwen runs of the default constructor.

There has been some discussion of allowing structs to have limited default 
constructors, but they would still disallow most of the kinds of things that 
people would want to do with a default constructor. init _must_ be known at 
compile time. So, its value is taken from the value that a struct gets when all 
of its member variables have been directly initialized (or default initialized) 
and no constructor has been called.

It _can_ be a bit annoying at times, but it's either that or allow undefined 
struct values, which would be a big problem and go against D's philosophy on 
default initialization.

Since you can't have a default constructor, what you do instead is define a 
static opCall() function for the struct and use that. Then you construct a 
struct (with the name S in this case) like so:

auto s = S();

You're still stuck with the init state if you do

S s;

or if you're dealing with structs in an array and the like (since they're all 
default constructed to the struct's init property). But you can do RAII just 
fine. Personally, I _always_ use this syntax when declaring struct variables:

auto S = S();

So, the situation isn't exactly ideal, but we're kind of stuck, given the 
various constraints that we have to work with.

- Jonathan M Davis

Sean Eskapp <eatingstaples gmail.com> writes:

== Quote from Jonathan M Davis (jmdavisProg gmx.com)'s article
 On Saturday 12 February 2011 10:48:04 Sean Eskapp wrote:
 With the removal of scope classes, the only way of which I know to use RAII
 objects is to use structs. This is fine, but structs aren't allowed default
 constructors! Why is this?

 It's because they have to have an init property. _All_ types have an init
 property. It's what they're default initialized to. For bool, it's false. For
 integral types, it's 0. For floating point types, it's NAN. Etc. For classes,
 that's null, so they can have default constructors just fine. However, for
 structs, it must be what the struct's member variables are directly initialized
 to. If you could have an arbitrary default constructor, then you'd have the
 problem of it throwing exceptions as well as it trying to run functions which
 could only be run at runtime. The struct's init value must be known at compile
 time, and it must _always_ be the same. So, it can't involve any functions
which
 would have to be run at runtime, and it can't involve anything that would
result
 in differing values betwen runs of the default constructor.
 There has been some discussion of allowing structs to have limited default
 constructors, but they would still disallow most of the kinds of things that
 people would want to do with a default constructor. init _must_ be known at
 compile time. So, its value is taken from the value that a struct gets when all
 of its member variables have been directly initialized (or default initialized)
 and no constructor has been called.
 It _can_ be a bit annoying at times, but it's either that or allow undefined
 struct values, which would be a big problem and go against D's philosophy on
 default initialization.
 Since you can't have a default constructor, what you do instead is define a
 static opCall() function for the struct and use that. Then you construct a
 struct (with the name S in this case) like so:
 auto s = S();
 You're still stuck with the init state if you do
 S s;
 or if you're dealing with structs in an array and the like (since they're all
 default constructed to the struct's init property). But you can do RAII just
 fine. Personally, I _always_ use this syntax when declaring struct variables:
 auto S = S();
 So, the situation isn't exactly ideal, but we're kind of stuck, given the
 various constraints that we have to work with.
 - Jonathan M Davis

Alright, thanks for clearing that up!