Why Does `make_integer_sequence` Cause Out-of-Memory Errors and How Can This Be Solved?

Why make_integer_sequence Runs Out of Memory

The C 14 make_integer_sequence is a versatile tool for constructing sequences of integers. However, its default implementation can run into memory issues, as demonstrated in the provided code. The error "virtual memory exhausted" occurs when the program requires more memory than the system can allocate.

Understanding the Root Cause

The root cause lies in the helper structure make_helper. It recursively expands itself until N is equal to 0. However, if N is large, this recursion can lead to an excessive number of template instantiations. The exponential growth of template instantiations and the memory required to hold them result in the out-of-memory error.

Solving the Memory Exhaustion Issue

To alleviate the memory exhaustion issue, one approach is to use a log N implementation. This method avoids the exponential recursion and reduces memory usage to the logarithmic scale.

Here's a sample log N implementation:

template<class T> using Invoke = typename T::type;

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> {};

This implementation avoids the exponential recursion by recursively splitting N in half until it becomes 0 or 1. The logarithmic time and space complexity ensures that no matter how large N is, the memory usage remains manageable.

In summary, the out-of-memory error when using make_integer_sequence arises from excessive template instantiations. Using a log N implementation, such as the one provided above, can mitigate this issue and allow for the creation of large sequences without encountering memory exhaustion.

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