absl/strings/has_absl_stringify.h

0001 // Copyright 2022 The Abseil Authors
0002 //
0003 // Licensed under the Apache License, Version 2.0 (the "License");
0004 // you may not use this file except in compliance with the License.
0005 // You may obtain a copy of the License at
0006 //
0007 //     https://www.apache.org/licenses/LICENSE-2.0
0008 //
0009 // Unless required by applicable law or agreed to in writing, software
0010 // distributed under the License is distributed on an "AS IS" BASIS,
0011 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
0012 // See the License for the specific language governing permissions and
0013 // limitations under the License.
0014
0015 #ifndef ABSL_STRINGS_HAS_ABSL_STRINGIFY_H_
0016 #define ABSL_STRINGS_HAS_ABSL_STRINGIFY_H_
0017
0018 #include <type_traits>
0019 #include <utility>
0020
0021 #include "absl/base/config.h"
0022 #include "absl/strings/string_view.h"
0023
0024 namespace absl {
0025 ABSL_NAMESPACE_BEGIN
0026
0027 namespace strings_internal {
0028
0029 // This is an empty class not intended to be used. It exists so that
0030 // `HasAbslStringify` can reference a universal class rather than needing to be
0031 // copied for each new sink.
0032 class UnimplementedSink {
0033  public:
0034   void Append(size_t count, char ch);
0035
0036   void Append(string_view v);
0037
0038   // Support `absl::Format(&sink, format, args...)`.
0039   friend void AbslFormatFlush(UnimplementedSink* sink, absl::string_view v);
0040 };
0041
0042 }  // namespace strings_internal
0043
0044 // `HasAbslStringify<T>` detects if type `T` supports the `AbslStringify()`
0045 // customization point (see
0046 // https://abseil.io/docs/cpp/guides/format#abslstringify for the
0047 // documentation).
0048 //
0049 // Note that there are types that can be `StrCat`-ed that do not use the
0050 // `AbslStringify` customization point (for example, `int`).
0051
0052 template <typename T, typename = void>
0053 struct HasAbslStringify : std::false_type {};
0054
0055 template <typename T>
0056 struct HasAbslStringify<
0057     T, std::enable_if_t<std::is_void<decltype(AbslStringify(
0058            std::declval<strings_internal::UnimplementedSink&>(),
0059            std::declval<const T&>()))>::value>> : std::true_type {};
0060
0061 ABSL_NAMESPACE_END
0062 }  // namespace absl
0063
0064 #endif  // ABSL_STRINGS_HAS_ABSL_STRINGIFY_H_