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Comment by stormbrew | Hacker News Reader

It's possible to argue here that the problem is not so much the order of type-then-name but the overloaded meanings of * and < at the grammar level.

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More generally this falls into a discussion of context-specific vs context-free grammars. Of which C++ falls into the former, Java falls into the latter.

The grammar of C++ is not a context-sensitive grammar. It's Turing-complete, on account of its template metaprogramming capabilities.

I'd be very surprised if Java's grammar were context-free. Do you have a source for this? I wasn't able to find one with a quick search.

It's Turing complete because it could simulate a Turing machine, not because of metaprogramming. The language brainfuck is Turing complete, for example.

Checking if a Brainfuck program is well formed (i.e. can be run) is a linear time operation. In C++ this can take forever. They have different complexities.

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.

No, the claim was that metaprogramming made the grammar of C++ Turing complete.

Metaprogramming in C++ is TC, but it's not what makes C++ TC by itself.

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.

Thanks, i see what you were saying now, i misunderstood your original comment.

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.

>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

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".

Yes i now realize that, thanks for explaining further. The original comment was worded in a way that i misunderstood the claim.

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.

It was a weird phrasing to me, but i get what you were all saying now.

Pretty much every programming language has a nicely parseable context-free "rough syntax" (my term I just invented) that can be written down formally for the language documentation and a parsing tool. And then every language also has a notion of "well-formed programs", which introduces a whole bunch of additional constraints on what programs should actually be accepted by compiler frontend.

Well-formedness includes type checking. But even without full type checking that can be done later, it also includes things like being aware, in C, of whether a given identifier is declared as a typedef in the current scope. So while C has a nice context-free "rough syntax" formally specified in the standard, its actual input language is context sensitive.

As for Java, the first example that comes to mind is that constructors must have the same name as the class they belong to. This "choose whatever identifier you like, but at some later point repeat that exact same identifier" is a very typical example of something that is not context-free.

You might disagree whether this constraint is part of what you consider Java's "grammar". So the answer to your question depends on what language level you are thinking of. But whichever level you apply to Java, you should apply the same to C++. C++ also has a context-free "rough syntax" in its standard.

https://stackoverflow.com/questions/14589346/is-c-context-fr...

Context free means something different to what you're saying here. This is a good discussion of this topic, I never knew C++ was so irregular and informal.

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