Re: Avoiding copying of an object

SG <>
Sun, 26 Sep 2010 03:01:08 -0700 (PDT)
On 26 Sep., 11:06, Adam Markiewicz wrote:

I'm designing a container class similar to those in STL. It is implemente=


as a template with two parameters: contained type and an allocator.

  template <typename Type, typename Allocator>
  class FixedArray

Now, to write a function that takes FixedArray as a param to support
different allocators I would need (example):

  template <typename Allocator>
  float average(const FixedArray<float, Allocator>& array);

However array is const, so Allocator wouldn't be needed at all. Is would =


much simpler to write:

  float average(const FixedArray<float>& array);

This syntax can be provided by specifying a suitable copy constructor, bu=


I want to avoid unnecessary copying overhead. Is there some other
clean solution to this problem?

Making 'average' a function template might not be a bad idea. What if
you also need it for T=double, T=complex<float>, etc? The STL-way of
writing such a function would be to make it a template that accepts a
pair of iterators. See std::accumulate.

Another solution would be to use inheritance:

   template<class T>
   class fixed_array_base
      std::size_t size() const {return end_ - begin_;}
      T & operator[](std::size_t idx)
      { assert(idx<size()); return begin_[idx]; }
      T const& operator[](std::size_t idx) const
      { assert(idx<size()); return begin_[idx]; }
      fixed_array_base() : begin_(0), end_(0) {}
      ~fixed_array_base() {}
      T* begin_;
      T* end_;
      fixed_array_base(fixed_array_base const&);
      fixed_array_base& operator=(fixed_array_base const&);

   template<class T, class Alloc = std::allocator<T> >
   class fixed_array : public fixed_array_base<T>
      explicit fixed_array(std::size_t n, Alloc const& a = Alloc());
      fixed_array(fixed_array const&);
      fixed_array& operator=(fixed_array const&);
      Alloc allocator_;

which could be used like this:

   float average(fixed_array_base<float> const& fa);

   float test() {
      fixed_array<float,custom_alloc> numbers (100);
      for (int k=0; k<100; ++k)
         numbers[k] = k;
      return averate(numbers);


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