c++ - why destructor is not called implicitly in placement new"?

Question

As referenced in this site... http://www.parashift.com/c++-faq-lite/dtors.html#faq-11.10 But i did not find the reason, why we should explicitly call the desturctor?

Answer 1

You can think of it as a call to delete, but since you used placement new, you don't want to use delete, as that would attempt to free the memory. If you wanted it to be called automatically, you could use RAII:

// Could use a templated version, or find an existing impl somewhere:
void destroy_fred(Fred* f) {
   f->~Fred();
}

void someCode()
{
   char memory[sizeof(Fred)];
   void* p = memory;
   boost::shared_ptr<Fred> f(new(p) Fred(), destroy_fred);

   // ...

   // No need for an explicit destructor, cleaned up even during an exception
}