Re: [boost] Re: crossbreed of boost::array & std::vector

From: David Abrahams (david.abrahams_at_[hidden])
Date: 2001-09-09 07:59:50

----- Original Message -----
From: "Peter Dimov" <pdimov_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Sunday, September 09, 2001 7:49 AM
Subject: Re: [boost] Re: crossbreed of boost::array & std::vector

> From: <rwgk_at_[hidden]>
> > However, what I am interested in is a vector with two pieces
> > of member data:
> >     T data_[N];
> >     std::size_t size_; // default constructor: size_ = 0
> >     std::size_t size() const { return size_; }
> >     std::size_t capacity() const { return N; }
>
> The C array will default-construct the elements. This is not compatible
with
> a std::vector. If you replace it with a char array, we're back to the
> alignment problem.

It is possible to fix the alignment problem or detect at compile-time that
we don't know how to fix it, though:

// something like this:
union everything
{
   int _int;
   double _double;
   // etc... examples of all POD types
};

template <std::size_t N>
struct pod_storage
{
    everything align; // wasted space
    char data[N];     // construct elements here
};

template <class T, std::size_t max>
struct stack_vector
{
 public: // interface
    //...
 private: // data members
    BOOST_STATIC_ASSERT(sizeof(everything) % boost::alignment_of<T> == 0);
    BOOST_STATIC_ASSERT(boost::alignment_of<T> < sizeof(everything));
    pod_storage<max * sizeof(T)> m_storage;
};