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Fix parameter type of formatter<char*> #3432

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merged 1 commit into from
May 20, 2023

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timsong-cpp
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Since this is macro expansion, const Type& expands to const Char*& instead of the desired Char* const&.

@vitaut
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vitaut commented May 13, 2023

Thanks for the PR but this specialization should be there at all, removed in aeedac5.

@vitaut vitaut closed this May 13, 2023
@timsong-cpp
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timsong-cpp commented May 13, 2023

this specialization should [not] be there at all

Hmm, are you sure? I think the result of this removal is that I can still format a char* but not a range of it (and std::formatter does provide this specialization).

Actually, never mind, a quick test suggests that it still works, interesting...

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OK, range_formatter works because it uses the mapper, but not fmt::join for either ranges or tuples. I'm not sure why this inconsistency (within fmt and also with std::formatter) is desirable.

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vitaut commented May 14, 2023

The char* formatter is redundant. The only reason it has a formatter in the standard is because there is no other ways to represent formattability there (yet).

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If I have a my::Optional<T> whose formatter wants to support all the specifiers supported by T, how should I write it so that it works for my::Optional<char*>?

Also as mentioned, this breaks fmt::join for both ranges and tuples: https://gcc.godbolt.org/z/45q54oe4T

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brevzin commented May 15, 2023

A simple wrapping example now breaks:

#include <fmt/format.h>

template <typename T>
struct wrapped {
    T t;
};

template <typename T>
struct fmt::formatter<wrapped<T>> {
    fmt::formatter<T> underlying;

    constexpr auto parse(auto& ctx) {
        return underlying.parse(ctx);
    }

    auto format(wrapped<T> const& w, auto& ctx) const {
        return underlying.format(w.t, ctx);
    }
};

auto f(wrapped<char*> w) -> std::string {
    return fmt::format("{}", w); // error: formatter<char*>::formatter() is deleted
}

@vitaut vitaut reopened this May 17, 2023
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vitaut commented May 17, 2023

Good point, reverted in 616a493. Let's reopen this.

@vitaut vitaut merged commit 08ef0d0 into fmtlib:master May 20, 2023
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vitaut commented May 20, 2023

merged, thanks

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3 participants