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Fix reference_wrapper ambiguity with format_as (#4434)

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This commit is contained in:
Jeremy Rifkin
2025-05-10 13:15:45 -05:00
committed by GitHub
parent 55a8f6a4be
commit 4999416e5c
2 changed files with 52 additions and 1 deletions
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21
include/fmt/std.h
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@ -678,9 +678,28 @@ 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

32
test/std-test.cc
View File

@ -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)));
}