pon., 16 paź 2023 o 11:37 Sebastian Wittmeier via Std-Proposals
<std-proposals_at_[hidden]> napisał(a):
>
> Hi Hadriel,
>
> I really have not known, whether it was working with the preprocessor, without reading up and trying out.
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> I think that topic belongs slightly together with the escaping/delimiter discussion and there are five basic options to activate the preprocessor:
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> A) Each (or only the more complex) C++ expression leave the F string and resume it, everything is concateated, e.g.:
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> print( X"SOME_FLAG = {" SOME_FLAG ":x}" );
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> - easier to include the preprocessor without preprocessing time parsing
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> - proliferation of quotes/unquotes
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> - looks more like the iostream way, when we add + or even << in between
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>
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> B) simple specific escaping
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> print( X"SOME_FLAG = { ####SOME_FLAG####:x }" );
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> - the part between #### #### is handled by the preprocessor, although it is within a X/R string
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> - also helps with colon
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> - possible to forget for generic code, except if #### required for all {} expressions, e.g. a l-value identifier could be a preprocessor redefined symbol
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> - so either it is required every time (no simple {lvalue} possible anymore) or it is only used, when needed and will be often forgotten or other code changes (SOME_FLAG is suddenly a preprocessor define) that would require it
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>
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> C) complete escaping
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> print( X"SOME_FLAG = { a ? \"\\\\\" \\: \"\\:\" :x }" ); // for a ? "\" : ":"
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> - certain characters are escaped: '"', '\', '}', ':'
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> - not easy to read
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> - better (?) than heuristics with additional paranthesis, which only sometimes work
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>
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> D) Flexible escaping similar to raw string literals
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> print( X"SOME_FLAG = {dch{ a ? b : c :dchx }dch" );
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> - with dch a delim character sequence, which can be freely chosen
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> E) have the preprocessor parse the string itself
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> print( X"SOME_FLAG = { SOME_FLAG:x}" );
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> - most difficult to sync WG21/WG14
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>

Another case to consider:
```
auto x = Obj{ true ? 1 : 2}; //valid code
auto y = X" test { Obj{ true ? 1 : 2} }"; //valid?
auto y = X" test { Obj{ (true ? 1 : 2)} }"; //valid?
auto y = X" test { (Obj{ true ? 1 : 2}) }"; //valid?
```

>
> -----Ursprüngliche Nachricht-----
> Von: Hadriel Kaplan <hkaplan_at_[hidden]>
> Gesendet: So 15.10.2023 18:45
> Betreff: Re: [std-proposals] Supporting f-strings in C++: draft-R1
> An: std-proposals_at_[hidden];
> CC: Sebastian Wittmeier <wittmeier_at_[hidden]>;
> > From: Std-Proposals <std-proposals-bounces_at_[hidden]> on behalf of Sebastian Wittmeier via Std-Proposals <std-proposals_at_[hidden]>
>
> > you write that there is no preprocessor support within the f-strings.
> > Is it possible to concatenate preprocessor macro results with preprocessor string concatenation?
> > If yes, would it be further possible to have the preprocessor preprocess code in its usual way, put it through a STRINGIFY macro and concatenate it into the f string?
>
> You can use preprocessor macros to stringify other macro expansions as you can do today, of course - but as far as I know the preprocessor cannot concatenate two string literals into one token literal using the `##`. It's usually the phase-6 translation right afterwards that concatenates them instead.
>
> So then I don't see how you'd get it to become an f-string, since presumably you'd want to join multiple stringified literals into one final f-string.
>
> For just _one_ stringified macro I think you can do it, but that doesn't seem useful.
>
> I.e., you might be able to get a stringified `__LINE__` for line 42 to become a `F"42"`, but if you wanted it to become `F"{42:x}"` instead, I don't think that's possible. (though I'm no PP expert)
>
> I mean basically think of the `F"..."` or `X"..."` as being similar to `R"(...)"` in terms of tokens, and you can try to create an `R"(...)"` using the preprocessor today to see if it's possible - I don't think it is, but I might be wrong. If it _is_ possible then it should be possible to do for f-strings too.
>
> ---
>
> Or maybe I misunderstand you - are you instead saying: "I know macro-magic won't work with f-strings as defined, but can the proposal define some way for it to work?"
>
> -hadriel
>
>
>
> Juniper Public
>
>
>
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