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Improve compile-time formatting #4118
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test/util.h
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// FOR_EACH macro that applies a given function to each variadic macro argument. | ||
// FOREACH macro helpers | ||
#define FOR_EACH_0(WHAT) | ||
#define FOR_EACH_1(WHAT, X) WHAT(X) | ||
#define FOR_EACH_2(WHAT, X, ...) WHAT(X), FOR_EACH_1(WHAT, __VA_ARGS__) | ||
#define FOR_EACH_3(WHAT, X, ...) WHAT(X), FOR_EACH_2(WHAT, __VA_ARGS__) | ||
#define FOR_EACH_4(WHAT, X, ...) WHAT(X), FOR_EACH_3(WHAT, __VA_ARGS__) | ||
#define FOR_EACH_5(WHAT, X, ...) WHAT(X), FOR_EACH_4(WHAT, __VA_ARGS__) | ||
#define FOR_EACH_6(WHAT, X, ...) WHAT(X), FOR_EACH_5(WHAT, __VA_ARGS__) | ||
#define FOR_EACH_7(WHAT, X, ...) WHAT(X), FOR_EACH_6(WHAT, __VA_ARGS__) | ||
#define FOR_EACH_8(WHAT, X, ...) WHAT(X), FOR_EACH_7(WHAT, __VA_ARGS__) | ||
#define FOR_EACH_9(WHAT, X, ...) WHAT(X), FOR_EACH_8(WHAT, __VA_ARGS__) | ||
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#define FOR_EACH_GET_MACRO(_1, _2, _3, _4, _5, _6, _7, _8, _9, NAME, ...) NAME | ||
#define FOR_EACH(action, ...) \ | ||
FOR_EACH_GET_MACRO(__VA_ARGS__ __VA_OPT__(, ) FOR_EACH_9, FOR_EACH_8, \ | ||
FOR_EACH_7, FOR_EACH_6, FOR_EACH_5, FOR_EACH_4, \ | ||
FOR_EACH_3, FOR_EACH_2, FOR_EACH_1, FOR_EACH_0) \ | ||
(action __VA_OPT__(, ) __VA_ARGS__) |
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Let's not introduce this and avoid new macros in tests.
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In this case getting rid of the macros that means duplicating all the tests when testing for a format function that returns an std::string_view
. I'm not sure which of two evils is better.
AFAIK, a constexpr function returning the result of a format as std::string_view
necessarily has to take all arguments as NTTP, since it needs to store the formatted result as a static constexpr
, i.e. one such constant per unique set of arguments. And the issue there is that we can't pass "bla"
as NTTP, it needs to be some kind of fixed_string
.
I can't figure out a way to do this conditionally per variadic NTTP. As soon as the compiler sees a const char[]
it's game over. Hence why I did the conversion of const char[]
by wrapping each template argument in a lambda call with this FOR_EACH
macro.
Would you prefer I duplicate some/all test cases?
Even better of course would be a way to make the compiler conert the const char[]
NTTP to fixed_string
automagically.
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Nevermind the above, I figured out a way to get rid of the FOR_EACH
and do the conditional NTTP conversion from a char array to a fixed_string
without macros.
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Also I don't think we need those string_view
/ NTTP tests since they are testing the same logic.
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I would agree, except I'm seeing different compile errors with the NTTP tests vs the existing ones (mainly missing constexpr
specifiers).
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Could you provide an example on godbolt?
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Sure: https://godbolt.org/z/eqorr8qWr. Note that there's a bunch of things this example doesn't support. It's just a MWE of one of the differences I saw.
Of course this could be due to something I implemented wrongly. E.g. formatting 42
uses a different call chain than uint64_t{42}
, and I might have been going down a bunch of paths trying to make everything work with NTTP. Might have confused myself at some point 😄.
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Thanks for the example. Looks like the only thing that is missing from the coverage is formatted_size
(https://godbolt.org/z/8411Eaoh3) so let's revert all the test changes here and add a call to it to test_format
, something like:
constexpr auto n = fmt::formatted_size(format, args...);
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Done. Also added a missing test case to catch the missing constexpr
in the FMT_FORMAT_AS
macro.
test/compile-test.cc
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#endif | ||
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#if defined(__cpp_constexpr) && (__cpp_constexpr >= 202211L) && \ | ||
defined(FMT_CONSTEXPR20) && FMT_USE_NONTYPE_TEMPLATE_ARGS |
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FMT_CONSTEXPR20
is always defined:
Lines 133 to 139 in fb07b37
#if FMT_USE_CONSTEVAL | |
# define FMT_CONSTEVAL consteval | |
# define FMT_CONSTEXPR20 constexpr | |
#else | |
# define FMT_CONSTEVAL | |
# define FMT_CONSTEXPR20 | |
#endif |
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Will remove it, the other checks guarantee C++23 anyway.
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By the way, do you feel there's any value in having a compile-time format to template <fmt::detail_exported::fixed_string format, nttp_arg... args>
constexpr std::string_view format(); I feel it's not entirely trivial to come up with this (due to not being able to pass The implementation would be more or less what I had in this Godbolt example: https://godbolt.org/z/eqorr8qWr. namespace fmt {
namespace detail {
template <typename T> struct nttp_arg {
template <typename U = T> constexpr nttp_arg(U const& arg) : arg(arg) {}
T arg;
};
template <typename T>
nttp_arg(const T&) -> nttp_arg<T>;
template <typename Char, size_t N>
nttp_arg(const Char (&arg)[N]) -> nttp_arg<fmt::detail_exported::fixed_string<Char, N>>;
} // namespace detail
// Can't be consteval due to llvm bug: https://github.com/llvm/llvm-project/issues/82994.
template <fmt::detail_exported::fixed_string format, fmt::detail::nttp_arg... args>
constexpr std::string_view format() {
static constexpr auto str = [] consteval {
using namespace fmt::literals;
constexpr auto compiled_format = operator""_cf<format>();
constexpr auto result_length =
fmt::formatted_size(compiled_format, (args.arg)...);
auto result = std::array<char, result_length>{};
fmt::format_to(result.data(), compiled_format, (args.arg)...);
return result;
}();
return std::string_view(str.data(), str.size());
}
} // namespace fmt |
This diff is really hard to review because it keeps changing and contains unrelated changes. Please split it into individual self-contained PRs. And I don't think we need anything to do with NTTP in the API. |
This PR improves compile-time formatting. More specifically:
FMT_CONSTEXPR(20)
specifiers were added.detail_exported::fixed_string
type is now formattable and has a UDL operator.fmt::format
functions now generate the same error as the runtime ones when a type is not formattable.write()
overload'senable_if
s to avoid ambiguity when a type is unformattable during compile-time formatting.std::string_view
. This requires some rather ugly macros, but they allow avoiding duplicating all those tests.