Why Does C 14's `make_integer_sequence` Implementation Cause Performance Bottlenecks?

Implementation C 14 make_integer_sequence: A Performance Bottleneck Explained

The C 14 alias template make_integer_sequence offers a convenient way to create class template integer_sequence. However, as evident in the provided code, implementing make_integer_sequence using a helper structure like make_helper can lead to performance issues.

The error message "virtual memory exhausted" during compilation indicates that the compiler has run out of memory during template instantiation. This is caused by the excessive recursion and memory consumption involved in the recursive helper structure.

Cause of the Error

The make_helper structure is implemented using template metaprogramming techniques, where the compiler recursively generates successive integer sequences through multiple levels of nesting. This level of nesting leads to exponential memory consumption as the number of integers in the sequence increases.

Resolving the Issue

To resolve this issue, a log N implementation that doesn't require increased maximum depth for template instantiations is suggested:

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

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

// Similar implementation for concat and gen_seq structures

This implementation uses a divide-and-conquer approach, reducing the template depth from N to log N.

Compilation Performance

Using the simplified test case, the log N implementation compiles significantly faster than the recursive helper structure with a greatly reduced memory consumption. These improvements make the implementation suitable for larger integer sequences without encountering memory exhaustion errors.

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