Is C Template Metaprogramming Turing-Complete at Compile Time?

C Templates: Turing-Complete at Compile Time

Question:

Can C template metaprogramming be employed for Turing-complete computations at compile time? Provide a non-trivial example demonstrating this capability.

Answer:

Yes, C template metaprogramming is Turing-complete. This means that, in theory, any computation that a Turing machine can perform can also be implemented using C templates.

Provided Example:

The provided code snippet implements a Turing machine in C 11 using template metaprogramming. It simulates a machine that reads an input string of 'x' and 'split' characters and doubles the number of 'x' characters in the output.

Explanation:

The provided code snippet uses a type list to represent the input string. Each rule in the transition table of the Turing machine is encoded as a specialized template class. The controller function uses these rules to advance the Turing machine state and update the input by matching the current state and input with the encoded rules.

Usefulness in Practice:

While this demonstrates the Turing-completeness of C templates, it's important to note that practical applications in this realm are limited. The complexity of such metaprogramming code can introduce maintenance challenges, and its performance can be hindered by the reliance on template instantiation at compile time.

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