Re: allocate memory 'inside' POD

From:
=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Wed, 4 Apr 2012 19:41:08 -0700 (PDT)
Message-ID:
<jli8so$6pb$1@dont-email.me>
Am 04.04.2012 19:21, schrieb goran.pusic@gmail.com:

However, what you want is a variable-element-number array, and people are
doing it on various implementations. In fact, you might oftentimes see std::string
implemented like that (only one buffer for all data, chars included + copy-on-write).

Here's an implementation that might work.

template<typename t>
struct vararr
{
   const size_t len;
   t data[1];

   static vararr* create(size_t s) { return new (s) vararr(s); }

   ~vararr() { std::for_each(p1(), p1()+(len-1),&destruct); }

   void operator delete(void* p, size_t cElems) { delete [] static_cast<char*>(p); }

private:

   t* p1() { return&data[1]; }

   vararr(size_t s) : len(s)
   {
     t* p = p1();
     t* pEnd = p1() + len - 1;
     try
     {
       while(p != pEnd)
       {
         new (p) t;


I strongly suggest ensure that the global placement new is called here
by writing instead

::new (p) t;

         p++;
       }
     }
     catch(...)
     {
       while (--p>= p1())
         destruct(*p);
       throw;
     }
   }

   static void destruct(t& item)
   {
     item.t::~t();
   }

   void* operator new(size_t s, size_t cElems)
   {
     return new char[s + sizeof t * (cElems-1)];


The grammar does not allow to provide a type-id without parentheses to
the sizeof operator:

sizeof unary-expression
sizeof ( type-id )

So you need to write "sizeof (t)" instead of "sizeof t"

   }
};


Above implementation is broken as of core language defect

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#429

The reason is that you have a placement allocation function, but your
deallocation function with the signature

void operator delete(void* p, size_t cElems)

According to [basic.stc.dynamic.deallocation] p2:

"If class T [..] does declare a member deallocation function named
operator delete with exactly two parameters, the second of which has
type std::size_t (18.2), then this function is a usual deallocation
function."

This leads to the problem described in CWG issue 429.

To fix this problem it seems better to make destructor and operator
delete private and use a tag type for proper discrimination. Then,
provide a public destroy function. E.g.:

struct stub {};

template<typename t>
struct vararr
{
  const size_t len;
  t data[1];

  static vararr* create(size_t s)
  {
    return new (s, stub()) vararr(s);
  }

  static void destroy(vararr* pv)
  {
    if (pv)
    {
      pv->~vararr();
      operator delete(pv, size_t(), stub());
    }
  }

private:

  ~vararr() { std::for_each(p1(), p1()+(len-1), &destruct); }

  void* operator new(size_t s, size_t cElems, stub)
  {
    return ::new char[s + sizeof(t) * (cElems-1)];
  }

  void operator delete(void* p, size_t cElems, stub)
  {
    ::delete [] static_cast<char*>(p);
  }

  //... As before

};

You can simplify even further when the tag type is used to transport the
relevant information directly. E.g.

struct size_tag
{
  const size_t n;
  size_tag(size_t n) : n(n) {}
};

template<typename t>
struct vararr
{
  const size_t len;
  t data[1];

  static vararr* create(size_t s)
  {
    return new (size_tag(s)) vararr(s);
  }

  static void destroy(vararr* pv)
  {
    if (pv)
    {
      pv->~vararr();
      operator delete(pv, size_tag(0));
    }
  }

private:

  ~vararr() { std::for_each(p1(), p1()+(len-1), &destruct); }

  void* operator new(size_t s, size_tag cElems)
  {
    return ::new char[s + sizeof(t) * (cElems.n-1)];
  }

  void operator delete(void* p, size_tag)
  {
    ::delete [] static_cast<char*>(p);
  }

  //... As before

};

HTH & Greetings from Bremen,

Daniel Kr?gler

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