Free Store

operator new [] adds a magic cookie to the beginning of arrays for which the number of elements will be needed by operator delete []. These are arrays of objects with destructors and arrays of objects that define operator delete [] with the optional size_t argument. This cookie can be examined from a program as follows:

typedef unsigned long size_t;
extern "C" int printf (const char *, ...);

size_t nelts (void *p)
{
  struct cookie {
    size_t nelts __attribute__ ((aligned (sizeof (double))));
  };

  cookie *cp = (cookie *)p;
  --cp;

  return cp->nelts;
}

struct A {
  ~A() { }
};

main()
{
  A *ap = new A[3];
  printf ("%ld\n", nelts (ap));
}

Linkage

The linkage code in g++ is horribly twisted in order to meet two design goals:

1) Avoid unnecessary emission of inlines and vtables.

2) Support pedantic assemblers like the one in AIX.

To meet the first goal, we defer emission of inlines and vtables until the end of the translation unit, where we can decide whether or not they are needed, and how to emit them if they are.

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