On Wednesday, 12 November 2014 at 19:15:24 UTC, John McFarlane
wrote:
On Wednesday, 12 November 2014 at 00:31:31 UTC, Jesse Phillips
wrote:
On Tuesday, 11 November 2014 at 20:53:51 UTC, John McFarlane
wrote:
I'm trying to write a struct template that uses
`insertInPlace`. However, it doesn't work with certain
template type / compiler combinations. Consider the following:
import std.range;
struct S { const int c; }
S[] a;
insertInPlace(a, 0, S());
With DMD64 D Compiler v2.066.1, I get the following error:
/usr/include/dmd/phobos/std/array.d(1013): Error: cannot
modify struct dest[i] S with immutable members
/usr/include/dmd/phobos/std/array.d(1079): Error: template
instance std.array.copyBackwards!(S) error instantiating
./d/my_source_file.d(12345): instantiated from here:
insertInPlace!(S, S)
I believe DMD is correct here and here is why:
While the function is called "insert" the operation is
actually an assignment. DMD initializes all arrays elements to
the default value so your array position 0 actually contains
an S already. This means the operation is equivalent to
auto b = S();
b = S();
Since S is a value type you're actually making a modification
'c' as stored in 'b'. The compiler is unable to prove that
there is no other reference to that same memory location
(though in this case the variable is on the stack and the
modification is local so such knowledge may be possible).
That makes sense. In the case that `c` is a class, do you think
I'd have any luck if I made it immutable?
The quick answer is that it doesn't help. DMD still doesn't like
me using insertInPlace. This is a little disappointing as
immutable is supposed to avoid the need for copying. Then again,
I guess that's a compiler - not language - level affordance.
Thanks again.
In the short term, could anybody suggest a `static if`
expression to determine whether I can copy the type to the
satisfaction of `copyBackwards`? I tried isMutable but that
didn't seem to work.
Thanks, John
You probably want std.traits.isAssignable
pragma(msg, isAssignable!(S, S)); // False
That's exactly what I was looking for. Thank you.
