THUAI6/CAPI/cpp/grpc/include/absl/strings/internal/string_constant.h

// Copyright 2020 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_INTERNAL_STRING_CONSTANT_H_
#define ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_

#include "absl/meta/type_traits.h"
#include "absl/strings/string_view.h"

namespace absl
{
    ABSL_NAMESPACE_BEGIN
    namespace strings_internal
    {

        // StringConstant<T> represents a compile time string constant.
        // It can be accessed via its `absl::string_view value` static member.
        // It is guaranteed that the `string_view` returned has constant `.data()`,
        // constant `.size()` and constant `value[i]` for all `0 <= i < .size()`
        //
        // The `T` is an opaque type. It is guaranteed that different string constants
        // will have different values of `T`. This allows users to associate the string
        // constant with other static state at compile time.
        //
        // Instances should be made using the `MakeStringConstant()` factory function
        // below.
        template<typename T>
        struct StringConstant
        {
        private:
            static constexpr bool TryConstexprEval(absl::string_view view)
            {
                return view.empty() || 2 * view[0] != 1;
            }

        public:
            static constexpr absl::string_view value = T{}();
            constexpr absl::string_view operator()() const
            {
                return value;
            }

            // Check to be sure `view` points to constant data.
            // Otherwise, it can't be constant evaluated.
            static_assert(TryConstexprEval(value), "The input string_view must point to constant data.");
        };

#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
        template<typename T>
        constexpr absl::string_view StringConstant<T>::value;
#endif

        // Factory function for `StringConstant` instances.
        // It supports callables that have a constexpr default constructor and a
        // constexpr operator().
        // It must return an `absl::string_view` or `const char*` pointing to constant
        // data. This is validated at compile time.
        template<typename T>
        constexpr StringConstant<T> MakeStringConstant(T)
        {
            return {};
        }

    }  // namespace strings_internal
    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_