• =?UTF-8?B?Ik5vcmRsw7Z3Ig==?= (5/5) Jan 12 2014 Is there a trait to check whether a type has value or reference
• qznc (9/14) Jan 13 2014 No, afaik.
• John Colvin (15/20) Jan 13 2014 That's a complicated question in D. Only simple cases fit one or
• Tobias Pankrath (2/7) Jan 13 2014 Adding to the other answers: std.traits.hasIndirections
• Orvid King (5/13) Jan 13 2014 If you just want to check if specifically it's a structure, you could
• Dicebot (2/9) Jan 13 2014 is(T == struct)
• anonymous (2/9) Jan 13 2014 is(T == struct)
• Orvid King (9/18) Jan 13 2014 Or that, yeah XD (For some reason I was thinking about ensuring that

=?UTF-8?B?Ik5vcmRsw7Z3Ig==?= <per.nordlow gmail.com> writes:

Is there a trait to check whether a type has value or reference 
semantics?

I need this in a template struct that adaptively (using static 
if) represent histogram bins as either a dense static array or a 
sparse associative array.

"qznc" <qznc web.de> writes:

On Sunday, 12 January 2014 at 20:16:15 UTC, Nordlöw wrote:
 Is there a trait to check whether a type has value or reference 
 semantics?

 I need this in a template struct that adaptively (using static 
 if) represent histogram bins as either a dense static array or 
 a sparse associative array.

No, afaik.

However, the question goes deeper than you might have though. You 
probably want to distuingish between structs+scalars vs classes. 
However, what about ref-arguments? Mutable arrays are struct 
values, but have reference semantics. Immutable-value arrays 
(e.g. string) have value semantics.

You could use one version as default and define special cases via 
some stuff you find in std.traits.

"John Colvin" <john.loughran.colvin gmail.com> writes:

On Sunday, 12 January 2014 at 20:16:15 UTC, Nordlöw wrote:
 Is there a trait to check whether a type has value or reference 
 semantics?

 I need this in a template struct that adaptively (using static 
 if) represent histogram bins as either a dense static array or 
 a sparse associative array.

That's a complicated question in D. Only simple cases fit one or 
the other exactly.

E.g.
A class always has reference semantics, but may contain any 
combination of reference and value semantic fields.
A struct has value semantics, but may contain any combination of 
reference and value semantic fields.
A slice has value semantics w.r.t. its length and start point, 
but reference semantics for its data.
A static array has value semantics.

For your problem you should look at the set of IsSomething traits 
in the table at the top of 
http://dlang.org/phobos-prerelease/std_traits.html as they will 
hopefully be sufficient.

"Tobias Pankrath" <tobias pankrath.net> writes:

On Sunday, 12 January 2014 at 20:16:15 UTC, Nordlöw wrote:
 Is there a trait to check whether a type has value or reference 
 semantics?

 I need this in a template struct that adaptively (using static 
 if) represent histogram bins as either a dense static array or 
 a sparse associative array.

Adding to the other answers: std.traits.hasIndirections

Orvid King <blah38621 gmail.com> writes:

If you just want to check if specifically it's a structure, you could
always check `__traits(compiles, T()) && if(typeof(T()) == T)` beware
however that this will evaluate to true if T is a class with a static
opCall who's return type is T.

On 1/13/14, Tobias Pankrath <tobias pankrath.net> wrote:
 On Sunday, 12 January 2014 at 20:16:15 UTC, Nordlöw wrote:
 Is there a trait to check whether a type has value or reference
 semantics?

 I need this in a template struct that adaptively (using static
 if) represent histogram bins as either a dense static array or
 a sparse associative array.

 Adding to the other answers: std.traits.hasIndirections

"Dicebot" <public dicebot.lv> writes:

On Monday, 13 January 2014 at 13:41:30 UTC, Orvid King wrote:
 If you just want to check if specifically it's a structure, you 
 could
 always check `__traits(compiles, T()) && if(typeof(T()) == T)` 
 beware
 however that this will evaluate to true if T is a class with a 
 static
 opCall who's return type is T.

is(T == struct)

"anonymous" <anonymous example.com> writes:

On Monday, 13 January 2014 at 13:41:30 UTC, Orvid King wrote:
 If you just want to check if specifically it's a structure, you 
 could
 always check `__traits(compiles, T()) && if(typeof(T()) == T)` 
 beware
 however that this will evaluate to true if T is a class with a 
 static
 opCall who's return type is T.

is(T == struct)

Orvid King <blah38621 gmail.com> writes:

On 1/13/14, anonymous <anonymous example.com> wrote:
 On Monday, 13 January 2014 at 13:41:30 UTC, Orvid King wrote:
 If you just want to check if specifically it's a structure, you
 could
 always check `__traits(compiles, T()) && if(typeof(T()) == T)`
 beware
 however that this will evaluate to true if T is a class with a
 static
 opCall who's return type is T.

 is(T == struct)

Or that, yeah XD (For some reason I was thinking about ensuring that
it was constructible, which, after reading the question again, isn't
actually needed, woops :D)

With that in mind, value type semantics would be `enum
hasValueSemantics(T) = is(T == struct) || is(T == enum) || is(T ==
union) || isArray!T;` Be aware however that this assumes you want
arrays to be considered to have value semantics due to their length
and base element being by-value.