How Can We Optimize C 14's `make_integer_sequence` Implementation to Avoid Memory Exhaustion?

Elaboration on C 14 make_integer_sequence Implementation

In C 14, the make_integer_sequence alias template was introduced to simplify the creation of the integer_sequence class template. To implement make_integer_sequence, a helper structure, make_helper, is employed.

template< class T, T... I> struct integer_sequence
{
    typedef T value_type;
    static constexpr size_t size() noexcept { return sizeof...(I) ; }

};

template< class T, T N>
using make_integer_sequence = integer_sequence< T, 0,1,2, ... ,N-1 >; // only for illustration.

Here's where the issue arises. In your implementation, the make_helper structure uses a recursive approach. While this may seem like a straightforward solution, it can lead to a rapid increase in template instantiations, particularly when the N parameter is large.

template< class T, T N, T... I >
struct make_helper
{
   typedef typename mpl::if_< T(0) == N,  
                  mpl::identity< integer_sequence<T,I...> >,
                  make_helper< T, N-1, N-1,I...> 
               >::type;
};

Due to this exponential growth, the compiler can quickly exhaust virtual memory, as you encountered when you changed the GEN macro to generate integer sequences with multiples of 4.

A Logarithmic Depth Implementation

The memory exhaustion issue can be addressed by employing a logarithmic depth implementation. This approach minimizes the number of recursive calls, reducing the template instantiation depth.

template<unsigned...> struct seq{ using type = seq; };

template<class S1, class S2> struct concat;

template<unsigned... I1, unsigned... I2>
struct concat<seq<I1...>, seq<I2...>>
  : seq<I1..., (sizeof...(I1)+I2)...>{};

template<class S1, class S2>
using Concat = Invoke<concat<S1, S2>>;

template<unsigned N> struct gen_seq;
template<unsigned N> using GenSeq = Invoke<gen_seq<N>>;

template<unsigned N>
struct gen_seq : Concat<GenSeq<N/2>, GenSeq<N - N/2>>{};

template<> struct gen_seq<0> : seq<>{};
template<> struct gen_seq<1> : seq<0>{};

In this logarithmic depth implementation, the recursion depth grows logarithmically with N. This significantly reduces the memory consumption and allows for the compilation of integer sequences with much larger values of N.

By employing the logarithmic depth implementation, the original issue of virtual memory exhaustion is resolved, allowing for the generation of integer sequences with large values of N.

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