It is easy to create an array of objects T that are uninitialized.
malloc would certainly work.
On Mon, Nov 2, 2009 at 2:18 AM, Andrew Haley <aph@xxxxxxxxxx> wrote:
> ?ukasz Lew wrote:
>
>> Is it possible to create array of objects T without calling the
>> default constructors T::T() ?
>> Calling other constructor instead would be great.
>> Leaving the memory uninitalize is ok as well, as I can do placement new
>> later.
>>
>> I want to avoid char* casting to T* because it breaks strict aliasing
>> rules
>> (stopping optimizations from happening and slowing my program by 25%.)
>>
>> Answer to this question would provide walkaround to the aliasing
>> problem I described few days ago.
>>
>> Thanks in advance for any ideas
>> Lukasz Lew
>>
>>
>> PS
>> Manual says that uinon is the way to go with strict-aliasing, but
>> unfortunately:
>>
>> union {
>> char tab [N * sizeof(Elt)];
>> Elt tab2 [N];
>> };
>>
>> Doesn't compile because Elt *has* default constructor.
>
> I don't really understand why you need this. character types are special
> in that they can alias all POD types. Is the problem here that you're
> trying to build a backing array of char type for a non-POD object?
>
> Andrew.
>
>
> 3.9 Types
>
> For any complete POD object type T, whether or not the object holds a
> valid value of type T, the underlying bytes (1.7) making up the object
> can be copied into an array of char or unsigned char. 36) If the
> content of the array of char or unsigned char is copied back into the
> object, the object shall subsequently hold its original value.
>
> [Example:
> #define N sizeof(T)
> char buf[N];
> T obj; // obj initialized to its original
> value
> memcpy(buf, &obj, N); // between these two calls to memcpy,
> // obj might be modified
> memcpy(&obj, buf, N); // at this point, each subobject of
> obj of scalar type
> // holds its original value
>
> 36) By using, for example, the library functions (17.4.1.2) memcpy or
> memmove.
>
