71 lines
2.5 KiB
C
71 lines
2.5 KiB
C
|
// 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.");
|
||
|
|
};
|
||
|
|
|
||
|
|
template <typename T>
|
||
|
|
constexpr absl::string_view StringConstant<T>::value; // NOLINT
|
||
|
|
|
||
|
|
// 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_
|