has_absl_stringify.h

// Copyright 2022 The Abseil Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef ABSL_STRINGS_HAS_ABSL_STRINGIFY_H_
#define ABSL_STRINGS_HAS_ABSL_STRINGIFY_H_

#include <type_traits>
#include <utility>

#include "absl/base/config.h"
#include "absl/strings/string_view.h"

namespace absl {
ABSL_NAMESPACE_BEGIN

namespace strings_internal {

// This is an empty class not intended to be used. It exists so that
// `HasAbslStringify` can reference a universal class rather than needing to be
// copied for each new sink.
class UnimplementedSink {
 public:
  void Append(size_t count, char ch);

  void Append(string_view v);

  // Support `absl::Format(&sink, format, args...)`.
  friend void AbslFormatFlush(UnimplementedSink* sink, absl::string_view v);
};

}  // namespace strings_internal

// `HasAbslStringify<T>` detects if type `T` supports the `AbslStringify()`
// customization point (see
// https://abseil.io/docs/cpp/guides/format#abslstringify for the
// documentation).
//
// Note that there are types that can be `StrCat`-ed that do not use the
// `AbslStringify` customization point (for example, `int`).

template <typename T, typename = void>
struct HasAbslStringify : std::false_type {};

template <typename T>
struct HasAbslStringify<
    T, std::enable_if_t<std::is_void<decltype(AbslStringify(
           std::declval<strings_internal::UnimplementedSink&>(),
           std::declval<const T&>()))>::value>> : std::true_type {};

ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_STRINGS_HAS_ABSL_STRINGIFY_H_