Re: size of array
On Oct 7, 2:59 pm, Mathias Gaunard <loufo...@gmail.com> wrote:
On 7 oct, 04:28, zaim...@zaimoni.com wrote:
But do they need to store the *exact* requested size of the
allocation
Yes, how are they supposed to know how many times to call the
destructor otherwise?
The requested size in bytes, that actually makes it to the global
operator new[] in a reimplementation, is allowed (but not required) to
have nonzero bytes of overhead to store such things. I don't see how
a current-standard compliant compiler could withstand user replacement
of operator new[]/operator delete[] without using this allowance.
Please recall that the signatures for global operator new[] are
void* operator new[](std::size_t) throw(std::bad_alloc);
void* operator new[](size_t,std::nothrow_t&) throw();
For
UserType* test_ptr = new UserType[5];
, the compiler sets up the five UserType default constructor
invocations completely independently of the global operator new[].
Not only is there is nothing preventing an implementation from telling
global operator new[] to allocate sizeof(size_t)+5*sizeof(UserType)
bytes, then using the leading sizeof(size_t) bytes to store the
constructor/destructor count and actually initializing test_ptr to
sizeof(size_t) bytes after the pointer obtained from global operator
new[] : this kind of manuevering is actually required to withstand
user replacement of operator new[] robustly.
(Or more overhead than sizeof(size_t), if that is needed for correct
alignment.)
work even after the user replaces global operator new.
A robust compiler could exploit this allowance to withstand user
replacement of global operator new[], by requesting sizeof(size_t)
+n*sizeof(T) bytes, where n is the array size and T is the type being
allocated. The count of how many tims to invoke the nontrivial
destructor could then be stored in the sizeof(size_t) bytes before the
actual pointer returned by operator new[] in source code.
As long as operator new[] and operator delete[] agree on what the
offset relative to the raw pointer when
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