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copied!<p>The 4 bytes most likely contains the total number of objects in the allocation so delete [] will be able to loop over all objects in the array calling their destructor..</p> <p>To get back the original address, you could keep a lookup-table keyed on address / 16, which stores the base address and length. This will enable you to find the original allocation. You need to ensure that your allocation+4 doesn't cross a 16-byte boundary, however.</p> <p>EDIT: I went ahead and wrote a test program that creates 50 objects with a destructor via new, and calls delete []. The destructor just calls printf, so it won't be optimized away.</p> <pre><code>#include <stdio.h> class MySimpleClass { public: ~MySimpleClass() {printf("Hi\n");} }; int main() { MySimpleClass* arr = new MySimpleClass[50]; delete [] arr; return 0; } </code></pre> <p>The partial disassembly is below, cleaned up to be a bit more legible. As you can see, VC++ is storing array count in the initial 4 byes.</p> <pre><code>; Allocation mov ecx, 36h ; Size of allocation call scratch!operator new test rax,rax ; Don't write 4 bytes if NULL. je scratch!main+0x25 mov dword ptr [rax],32h ; Store 50 in first 4 bytes add rax,4 ; Increment pointer by 4 ; Free lea rdi,[rax-4] ; Grab previous 4 bytes of allocation mov ebx,dword ptr [rdi] ; Store in loop counter jmp StartLoop ; Jump to beginning of loop Loop: lea rcx,[scratch!`string' (00000000`ffe11170)] ; 1st param to printf call qword ptr [scratch!_imp_printf; Destructor StartLoop: sub ebx,1 ; Decrement loop counter jns Loop ; Loop while not negative </code></pre> <p>This book keeping is distinct from the book keeping that malloc or HeapAlloc do. Those allocators don't care about objects and arrays. They only see blobs of memory with a total size. VC++ can't query the heap manager for the total size of the allocation because that would mean that the heap manager would be bound to allocate a block exactly the size that you requested. The heap manager shouldn't have this limitation - if you ask for 240 bytes to allocate for 20 12 byte objects, it should be free to return a 256 byte block that it has immediately available.</p>

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