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Subject: Re: [ub] *** GMX Spamverdacht *** Re: type punning through congruent base class?
From: Jeffrey Yasskin (jyasskin_at_[hidden])
Date: 2014-01-16 16:53:00


On Thu, Jan 16, 2014 at 2:06 PM, Jens Maurer <Jens.Maurer_at_[hidden]> wrote:
> On 01/16/2014 10:10 PM, Herb Sutter wrote:
>> And dare I ask: Um, what about using part of the same allocated
>> buffer to hold a T and another part to hold a U, such as malloc(1000)
>> and read/write an int at offset 10 and read/write a short at offset
>> 314 -- they're the same allocation, so do we even have any way to
>> talk about that?
>
> They're the same allocation, but different regions of storage, so
> you're fine, in my opinion. (Implementing your own sub-allocators
> with a malloc backend probably depends on this to work.)

Mhmm; that matches my understanding. For some more litmus tests, how
about an object like:

struct S {
  int x;
  std::aligned_union<1, float>::type y;
};

If I write:
  S* s = (S*)malloc(sizeof(S));
  float* y = (float*)(&s->y);
  *y = 5.0f;

Do I have defined behavior? Can I use *y as an object of type float,
or does it stay an object of type std::aligned_union<float>::type? (Am
I using "object" in the right way here?)

If I change that to:

  S* s = new S;
  float* y = (float*)(&s->y);
  *y = 5.0f;

does anything change?

What if I use

  S s;
  float* y = (float*)(&s.y);
  *y = 5.0f;

instead? My reading of C suggests that moving the object on to the
stack makes a difference because the object now has a "declared type",
but what do we want in C++?

Or, finally

  S s;
  new (&s.y) float();
  float* y = (float*)(&s.y);

Types like llvm::SmallVector
(http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/SmallVector.h?revision=190708&view=markup#l112)
do this one.

If some part of the answer is that std::aligned_union is "magic", does
that make the possible implementation at
http://en.cppreference.com/w/cpp/types/aligned_storage#Possible_implementation
incorrect?

Thanks,
Jeffrey


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