[Issue 14194] New: nothrow emplace for classes needed

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=14194

          Issue ID: 14194
           Summary: nothrow emplace for classes needed
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: Phobos
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

Currently they are 5 "kinds" of emplace:

0) pure nothrow @safe T* emplace(T)(T* chunk);
1) T* emplace(T, Args...)(T* chunk, auto ref Args args)
     if (!is(T == struct) && Args.length == 1); 
2) T* emplace(T, Args...)(T* chunk, auto ref Args args)
     if (is(T == struct));
3) T emplace(T, Args...)(void[] chunk, auto ref Args args)
     if (is(T == class));
4) T* emplace(T, Args...)(void[] chunk, auto ref Args args)
     if (!is(T == class));

"Kind" 0, 1, 2 and 4 are documented as working on "non-class type". However,
except for 4, their signature doesn't reflect that, so they are pretty easy to
misuse, leading to unrelated error message (E.g:
/usr/include/dlang/dmd/std/conv.d(3839): Error: static assert  "Condition
cannot be emplaced from a Mutex."). (Condition and Mutex in their respective
modules in  core.sync).

That left us with only kind 3 to emplace classes, which means that a generic
approach to a memory allocator should use the void[] approach for it's
allocations.

Now, since it's a memory allocator, you want to have it nothrow, but kind 3
calls testEmplaceChunk, which itself check for the size and throw on error.

So far, it looks to me like calling emplace for a class with a void[] that's
too small is a programmer error, as he failed to create to correct size /
validate the arguments, and so it should be an in contract in emplace, rather
than a private function that throw exception.
Will try to submit something when I get a bit of time.

--