delete after new [] : with or without user destructor

Question

Deleting a pointer to an array allocated with new [] is "undefined behaviour". I'm just curious why the first delete in the following code leads to heap corruption IF the destructor is defined, otherwise nothing happens (as I humbly expected). Tested with Visual Studio 8 and GNU GCC version 4.7.2 (http://www.compileonline.com/compile_cpp11_online.php).

struct A
{
   //~A() {}
};

int main()
{
   A* a = new A[10];
   delete a;    //  heap corruption when the user destructor is defined

   int *b = new int[100];
   delete b;    //  no heap corruption
};

Answer 1

UB is not something you can rationalise about within the scope of the language, and at least without detailed knowledge of the toolchain, optimisation level, operating system, architecture and processor.

At a guess, though: with no destructor to call, the compiler is simply not generating any code that makes your code's fault visible in the real world. Such a destructor call may be hitting the header of your dynamically allocated memory block, which doesn't exist in the single new A example.