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digitalmars.D.bugs - [Issue 9528] New: std.array.appender can't append elements with const members

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=9528

           Summary: std.array.appender can't append elements with const
                    members
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: hsteoh quickfur.ath.cx



Code:
------------------------------------------
import std.array;

E[] fastCopy(E)(E[] src) {
        auto app = appender!(const(E)[])();
        foreach (i, e; src)
                app.put(e);
        return app.data;
}

E[] slowCopy(E)(E[] src) {
        E[] result;
        foreach (i, e; src)
                result ~= e;
        return result;
}

void main() {
        class C {}
        struct S { const(C) c; }
        S[] s = [ S(new C) ];

        //auto t = fastCopy(s); // Does not compile
        auto t = slowCopy(s);
}
------------------------------------------

If fastCopy is used in place of slowCopy, dmd git head gives:
------------------------------------------
/usr/src/d/phobos/std/array.d(2256): Error: cannot modify struct
(cast(S*)(*this._data).arr)[len] S with immutable members
test.d(6): Error: template instance
std.array.Appender!(const(S)[]).Appender.put!(S) error instantiating
test.d(22):        instantiated from here: fastCopy!(S)
test.d(7): Error: cannot implicitly convert expression (app.data()) of type
const(S)[] to S[]
test.d(22): Error: template instance test.fastCopy!(S) error instantiating
------------------------------------------

Is there any workaround for this? What I'm trying to accomplish is to copy an
array of elements with const fields, but selectively skip elements based on
some predicate (so straight .dup is out of the question). But using =~ is slow
because of continual resizing/reallocation.

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Feb 17 2013
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=9528


monarchdodra gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rswhite4 googlemail.com



*** Issue 9667 has been marked as a duplicate of this issue. ***

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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monarchdodra gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |monarchdodra gmail.com



The problem is that appender is making the double assuption that unqual implies
assignability, and that copyiable implies assignability (empace ony requires
copyability). Finally, it makes the wrong assumption that you can call opAssign
on something that is not yet initialized (append makes raw allocations).
Amongst a few other bugs mind you.

There is a lot of broken in appender that I've tried to fix before, but it is
very tricky because:
1) It is a ritical function that is used for strings, and needs to support
CTFE.
2) Anything that fixes emplace must not slow it down.
3) It should be calling emplace instead of opAssign but emplace is currently
broken for exactly the same reasons!

I should maybe try to fix it in smaller incremental steps, but it is very hard
to knowingly deliver code that you know is broken, and know how to fix.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=9528






 The problem is that appender is making the double assuption that unqual implies
 assignability, and that copyiable implies assignability (empace ony requires
 copyability). Finally, it makes the wrong assumption that you can call opAssign
 on something that is not yet initialized (append makes raw allocations).
 Amongst a few other bugs mind you.
 
 There is a lot of broken in appender that I've tried to fix before, but it is
 very tricky because:
 1) It is a ritical function that is used for strings, and needs to support
 CTFE.
 2) Anything that fixes emplace must not slow it down.
 3) It should be calling emplace instead of opAssign but emplace is currently
 broken for exactly the same reasons!
 
 I should maybe try to fix it in smaller incremental steps, but it is very hard
 to knowingly deliver code that you know is broken, and know how to fix.


Possible solution for this:

static if (__traits(compiles, { _data.arr.ptr[len] = cast(Unqual!T) item; })) {
    _data.arr.ptr[len] = cast(Unqual!T) item;
} else {
    memcpy(&_data.arr.ptr[len], cast(Unqual!(T)*) &item, T.sizeof);
}

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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monarchdodra gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs eml.cc



*** Issue 10753 has been marked as a duplicate of this issue. ***

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Aug 28 2013
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=9528




https://github.com/D-Programming-Language/phobos/pull/1529

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