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Fix reference_wrapper ambiguity with format_as #4434

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Merged
vitaut merged 4 commits into from
May 10, 2025
Merged

Fix reference_wrapper ambiguity with format_as #4434

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862b400
Fix reference_wrapper ambiguity with format_as
jeremy-rifkin May 3, 2025
9987081
Add another test
jeremy-rifkin May 10, 2025
a8b3702
clang-format
jeremy-rifkin May 10, 2025
19b8041
Move to detail namespace
jeremy-rifkin May 10, 2025
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21 changes: 20 additions & 1 deletion include/fmt/std.h
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Original file line number Diff line number Diff line change
Expand Up @@ -713,9 +713,28 @@ template <typename T, typename Char> struct formatter<std::complex<T>, Char> {
}
};

namespace detail {
template <typename T, typename Enable = void>
struct has_format_as : std::false_type {};
template <typename T>
struct has_format_as<T, void_t<decltype(format_as(std::declval<const T&>()))>>
: std::true_type {};

template <typename T, typename Enable = void>
struct has_format_as_member : std::false_type {};
template <typename T>
struct has_format_as_member<
T, void_t<decltype(formatter<T>::format_as(std::declval<const T&>()))>>
: std::true_type {};
} // namespace detail

// Guard against format_as because reference_wrappers are implicitly convertible
// to T&
template <typename T, typename Char>
struct formatter<std::reference_wrapper<T>, Char,
enable_if_t<is_formattable<remove_cvref_t<T>, Char>::value>>
enable_if_t<is_formattable<remove_cvref_t<T>, Char>::value &&
!detail::has_format_as<T>::value &&
!detail::has_format_as_member<T>::value>>
: formatter<remove_cvref_t<T>, Char> {
template <typename FormatContext>
auto format(std::reference_wrapper<T> ref, FormatContext& ctx) const
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32 changes: 32 additions & 0 deletions test/std-test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -414,4 +414,36 @@ TEST(std_test, format_shared_ptr) {
TEST(std_test, format_reference_wrapper) {
int num = 35;
EXPECT_EQ("35", fmt::to_string(std::cref(num)));
EXPECT_EQ("35", fmt::to_string(std::ref(num)));
EXPECT_EQ("35", fmt::format("{}", std::cref(num)));
EXPECT_EQ("35", fmt::format("{}", std::ref(num)));
}

// Regression test for https://github.com/fmtlib/fmt/issues/4424
struct type_with_format_as {
int x;
};

int format_as(const type_with_format_as& t) { return t.x; }

TEST(std_test, format_reference_wrapper_with_format_as) {
type_with_format_as t{20};
EXPECT_EQ("20", fmt::to_string(std::cref(t)));
EXPECT_EQ("20", fmt::to_string(std::ref(t)));
EXPECT_EQ("20", fmt::format("{}", std::cref(t)));
EXPECT_EQ("20", fmt::format("{}", std::ref(t)));
}

struct type_with_format_as_string {
std::string str;
};

std::string format_as(const type_with_format_as_string& t) { return t.str; }

TEST(std_test, format_reference_wrapper_with_format_as_string) {
type_with_format_as_string t{"foo"};
EXPECT_EQ("foo", fmt::to_string(std::cref(t)));
EXPECT_EQ("foo", fmt::to_string(std::ref(t)));
EXPECT_EQ("foo", fmt::format("{}", std::cref(t)));
EXPECT_EQ("foo", fmt::format("{}", std::ref(t)));
}