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by battery_cowboy 2241 days ago
The original comment was about Turing completeness, and it was defined incorrectly. I was giving an example of a dead-simple language that was Turing complete, because the claim was that metaprogramming made C++ Turing complete.
2 comments
wtetzner 2241 days ago
The claim wasn’t that C++ is Turing complete, that’s trivially true. The claim was that C++’s grammar is Turing complete. I don’t know if that’s exactly the right way to phrase it, but C++’s template expansion stuff is Turing complete.
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battery_cowboy 2240 days ago
It was a weird phrasing to me, but i get what you were all saying now.
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tom_mellior 2241 days ago
No, the claim was that metaprogramming made the grammar of C++ Turing complete.
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battery_cowboy 2241 days ago
Metaprogramming in C++ is TC, but it's not what makes C++ TC by itself.
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tom_mellior 2241 days ago
Yes, but it is the difference to other programming languages.

In C you cannot encode a Turing machine that is executed by the compiler at compile time. In Brainfuck you cannot encode a Turing machine that is executed by the compiler at compile time. In C++ you can encode a Turing machine that is executed by the compiler at compile time.

That is the difference we are discussing here.

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battery_cowboy 2240 days ago
Yes i now realize that, thanks for explaining further. The original comment was worded in a way that i misunderstood the claim.
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benibela 2241 days ago
>In C you cannot encode a Turing machine that is executed by the compiler at compile time.

But you can get quite close with macros

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tom_mellior 2240 days ago
No. You would need the ability to write unbounded loops or unbounded recursion. You don't have that with the C preprocessor.

Yes, you can do a lot with the C preprocessor. You can also do a lot in languages that only have bounded loops and are therefore not Turing complete. You can either express nonterminating computations (Turing completeness), or you can't (still powerful, but dramatically less poweful). This question is binary. There is no fuzziness, there is no approximation, there is no "quite close".

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MaxBarraclough 2241 days ago
You misunderstand. C++ is Turing-complete at compile time, due to template metaprogramming. This demonstrates that it isn't a context-free grammar.

This isn't true of all programming languages.

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battery_cowboy 2240 days ago
Thanks, i see what you were saying now, i misunderstood your original comment.
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