[boost] Improving efficiency of multi_array

From: Korcan Hussein (korcan_h_at_[hidden])
Date: 2006-12-01 05:24:56

I had a look into multi_array's implementation and i can see a number of 
opportunities to improve  the efficiency multi_array perhaps by quite a lot 
with only very minor changes.

Why are we not taking advantage of the fact that that 
allocation/de-allocation is separate from construction/destruction? i see in 
a number of places that multi_array is redundantly default constructing 
elements (like in multi_array::allocate_space) and then copying new elements 
into the array for example in multi_array i see a lot of:

allocate_space();  // allocate memory then default construct
std::copy(rhs.begin(),rhs.end(),this->begin());

instead why not just do an in-place copy construction from the outset, 
something like:

allocate_space(); // a new version which only allocates memory, no 
construction
std::uninitialized_copy(rhs.begin(), rhs.end(), this->begin());

When i create a multi_array i want to be able to "reserve" the shape of  a 
multi_array and then incrementally add new elements via in-place copy 
construction or even better using in-place factories.

We do not even need to explicitly store the size if we take advantage of 
uninitialized and initialized elements, just like how modern implementations 
of std::vector do.

Another minor thing is to apply the empty member/base optimizations 
(EMO/EBO) for allocator types which are empty in size.

So i would like to know the rationale for these, since i need something like 
multi_array i may just make the adjustments myself.

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