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).
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
