/pimpl_ptr

The C++11 way of implementing the Fast Pimpl Idiom

Primary LanguageC++

Helper class for the Fast Pimpl Idiom

The Pimpl Idiom is a great way to reduce compilation dependencies.

However, you introduce an additional indirection, which leads you performance penalties. There is a hack that stores the internal data into a char array. Look at GotW #28 why this is a bad idea.

How does the solution look like?

The typical implementation of the pimpl idiom looks like the following:

class C {
  struct impl;
  std::unique_ptr<impl> pimpl;
public:
  test();
  test(test const&);
 ~test();
  test& operator=(test const&);
};

The functions need to be defined in the implementation file where C::impl is known.

Sometimes, performance really matters and you don't want this indirection. Then, you want to reserve exactly as many bytes as the impl class would use. Here is how it looks like using the pimpl_ptr:

class C {
  struct impl;
  pimpl_ptr<impl, /* size to be reserved */> pimpl;
public:
  test();
  test(test const&);
 ~test();
  test& operator=(test const&);
};

pimpl_ptr can be declared without knowing about the implementation of impl but can't be constructed, destructed, copied or moved. So not declaring for example a copy constructor results in a compiler error. Also, giving the wrong /* size to be reserved */, a static_assert fails and get another compiler error.

Opaque types in C++11

C++11 introduces a template called std::aligned_storage. This allowes you to align the impl data in a standard-conformant way.

Here is how the pimpl_ptr could be defined:

template <typename T, std::size_t Size, int Align>
class pimpl_ptr {
  typename std::aligned_storage<Size, Align>::type data;

public:
  template <typename... Args>
  pimpl_ptr(Args&&... args)
  { new (&data) T(std::forward<Args>(args)...); }
  pimpl_ptr(pimpl_ptr const& o) { new (&data) T(*o); }
  pimpl_ptr(pimpl_ptr&  o)      { new (&data) T(*o); }
  pimpl_ptr(pimpl_ptr&& o)      { new (&data) T(std::move(*o)); }
 ~pimpl_ptr() { reinterpret_cast<T*>(&data)->~T(); }

  pimpl_ptr& operator=(pimpl_ptr const& o) { **this = *o; return *this; }
  pimpl_ptr& operator=(pimpl_ptr&& o)      { **this = std::move(*o); return *this; }

  T      & operator*()        { return *reinterpret_cast<T      *>(&data); }
  T const& operator*()  const { return *reinterpret_cast<T const*>(&data); }
  T      * operator->()       { return &**this; }
  T const* operator->() const { return &**this; }

  friend inline void swap(pimpl_ptr& lhs, pimpl_ptr& rhs)
  { using std::swap; swap(*lhs, *rhs); }
};

In fact, the whole pimpl_ptr is a little bit more complicated. First of all, every function has its own static_assert to ensure that Size and Align was supplied right. Furthermore, Align is made an optional parameter.

You can enjoy the full code there: pimpl_ptr.hh.

When should I use this class?

The pimpl_ptr encapsulates a hack. And it still is a hack. But it's a hack made safe.

Using impl itself needs some effort. Switching to from std::unique_ptr to pimpl_ptr only involves minor changes.

The good thing is, that you can't make anything wrong if it compiles. However, it will very often break compiling.

In sum, you can trade development and porting effort for runtime performance. Very often, this is not what you want. So decide wisely.

Is there a minimal working example?

Yes of course, feel free to look around in the repository.