Re: [boost] Vector with compile-time polymorphism

On 4/9/21 3:13 PM, Ivica B via Boost wrote:

Hi!

I am doing a performance C++ work and there is a data structure that we use and I think it is very useful even for the general public. Here is the description:

Currently, I call it *multivector* (I know there is a data structure with the same name, this is a temporary name). It works like this:

You specify all the types you want to hold in your multivector:

multivector mvect;

Each type is held in a separate std::vector for that type. So in the above case, multivector consists of three std::vectors: std::vector<rectangle>, std::vector<circle> and std::vector<triangle>.

Now, the magic happens when we use templates to process all three vectors at once. Imagine there is a function *draw* for all three types. We can do something like this:

mvect.for_each([&](auto& shape) { shape.draw(bitmap); });

The above code generates three for_each loops, for rectangle, circle and triangle. Inside the loops it calls the draw method for each type. There is no dispatching inside the loops, and the loops are really easy for the compiler to optimize (due to their simplicity).

Prototype implementation: https://github.com/ibogosavljevic/johnysswlab/blob/master/2021-02-virtualfun...

Advantages over std::vectorstd::variant>: * Uses less memory (std::variant uses the amount of memory per element that is big enough to store the biggest type) * Is more optimal: each type generates its own processing loop, there is no dispatching code inside the loop. The compilers vectorized these loops easier.

Downsides: * You cannot sort the elements in the whole vectors (even though you can sort them in individual containers) * Code bloat (many types can create a lot of code, this is the case with std::vectorstd::variant> too)

Please let me know what you think.

This sounds a lot like Boost.Fusion to me: https://www.boost.org/doc/libs/1_75_0/libs/fusion/doc/html/ template< typename... Types > using multivector = boost::fusion::vector< std::vector< Types >...

;

multivector mvect; boost::fusion::for_each(mvect, [&](auto& shape) { shape.draw(bitmap); });