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// 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.");
};
// 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_
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