On 11/20/2016 03:42 AM, ag0aep6g via Digitalmars-d-learn wrote:
On 11/20/2016 04:34 AM, Charles Hixson via Digitalmars-d-learn wrote:
Whether you would call the change "break things for your code" might be
dubious. It would be effectively broken, even if technically my code
was doing the correct thing. But my code wouldn't be storing the data
that needed storing, so effectively it would be broken.
I don't see how it's dubious. It's an error by the user. When users
are given a dynamic array (and not by reference), they cannot expect
that your code sees changes to length. That's just not how arrays
work. When a user has that wrong expectation, and writes wrong code
because of it, then it's arguably their own fault. However, if you
want you can hold their hand a bit and make the mistake less likely.
"Write something
for yourself" is what I'd like to do, given that the language doesn't
have that built-in support, but I can't see how to do it.
Wrap the array in a struct that has indexing, but doesn't allow
setting the length or appending. Here's a quick prototype:
----
struct ConstLengthArray(E)
{
private E[] data;
this(E[] arr) { this.data = arr; }
ref inout(E) opIndex(size_t i) inout { return data[i]; }
@property size_t length() const { return data.length; }
}
void main()
{
auto cla = ConstLengthArray!ubyte([1, 2, 3, 4, 5]);
/* Mutating elements is allowed: */
cla[0] = 10;
assert(cla[0] == 10);
/* No setting length, no appending: */
static assert(!__traits(compiles, cla.length = 3));
static assert(!__traits(compiles, cla ~= 6));
}
----
You might want to add support for slicing, concatenation, etc. Maybe
allow implicit conversion to const(E[]), though that would also allow
conversion to const(E)[] and that has a settable length again
Thinking it over a bit more, the item returned would need to be a
struct, but the struct wouldn't contain the array, it would just contain
a reference to the array and a start and end offset. The array would
need to live somewhere else, in the class (or struct...but class is
better as you don't want the array evaporating by accident) that created
the returned value. This means you are dealing with multiple levels of
indirection, so it's costly compared to array access, but cheap compared
to lots of large copies. So the returned value would be something like:
struct
{
private:
/** this is a reference to the data that lives elsewhere. It
should be a pointer, but I don't like the syntax*/
ubyte[] data;
int start, end; /// first and last valid indicies into data
public:
this (ubyte[] data, int start, int end)
{ this.data = data; this.start = start; this.end = end;}
...
// various routines to access the data, but to limit the access to
the spec'd range, and
// nothing to change the bounds
}
Which is really the answer you already posted, but just a bit more
detail on the construct, and what it meant. (Yeah, I could allow types
other than ubyte as the base case, but I don't want to. I'm thinking of
this mainly as a means of sharing a buffer between applications where
different parts have exclusive access to different parts of the buffer,
and where the buffer will be written to a file with a single fwrite, or
since the underlying storage will be an array, it could even be
rawwrite). I don't want to specify any more than I must about how the
methods calling this will format the storage, and this means that those
with access to different parts may well use different collections of
types, but all types eventually map down to ubytes (or bytes), so ubytes
is the common ground. Perhaps I'll need to write inbuffer,outbuffer
methods/wrappings, but that's far in the future.
P.S.: The traits that I mentioned previously were those given by:
static assert(!__traits(compiles, cla.length = 3));
static assert(!__traits(compiles, cla ~= 6));
in your main routine. I assumed that they were validity tests. I don't
understand why they were static. I've never happened to use static
asserts, but I would assume that when they ran cla wouldn't be defined.
N.B.: Even this much is just thinking about design, not something I'll
actually do at the moment. But this is a problem I keep coming up
against, so a bit of thought now seemed a good idea.
