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/*
* Copyright 2018 Google
*
* 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
*
* http://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 FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_TYPE_TRAITS_H_
#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_TYPE_TRAITS_H_
#if __OBJC__
#import <objc/objc.h> // for id
#endif
#include <type_traits>
namespace firebase {
namespace firestore {
namespace util {
#if __OBJC__
/**
* A type trait that identifies whether or not the given pointer points to an
* Objective-C object.
*
* is_objective_c_pointer<NSObject*>::value == true
* is_objective_c_pointer<NSArray<NSString*>*>::value == true
*
* // id is a dynamically typed pointer to an Objective-C object.
* is_objective_c_pointer<id>::value == true
*
* // pointers to C++ classes are not Objective-C pointers.
* is_objective_c_pointer<void*>::value == false
* is_objective_c_pointer<std::string*>::value == false
* is_objective_c_pointer<std::unique_ptr<int>>::value == false
*/
template <typename T>
struct is_objective_c_pointer {
private:
using yes_type = char (&)[10];
using no_type = char (&)[1];
/**
* A non-existent function declared to produce a pointer to type T (which is
* consistent with the way Objective-C objects are referenced).
*
* Note that there is no definition for this function but that's okay because
* we only need it to reason about the function's type at compile type.
*/
static T Pointer();
static yes_type Choose(id value);
static no_type Choose(...);
public:
using value_type = bool;
enum { value = sizeof(Choose(Pointer())) == sizeof(yes_type) };
constexpr operator bool() const {
return value;
}
constexpr bool operator()() const {
return value;
}
};
// Hard-code the answer for `void` because you can't pass arguments of type
// `void` to another function.
template <>
struct is_objective_c_pointer<void> : public std::false_type {};
#endif // __OBJC__
} // namespace util
} // namespace firestore
} // namespace firebase
#endif // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_TYPE_TRAITS_H_
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