unique_ptr.h

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/*
  Copyright (c) 2016, 2024, Oracle and/or its affiliates.
 
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  it under the terms of the GNU General Public License, version 2.0,
  as published by the Free Software Foundation.
 
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  as designated in a particular file or component or in included license
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  permission to link the program and your derivative works with the
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  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
 
  You should have received a copy of the GNU General Public License
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*/
 
#ifndef MYSQL_HARNESS_UNIQUEPTR_INCLUDED
#define MYSQL_HARNESS_UNIQUEPTR_INCLUDED
 
#include <assert.h>  // <cassert> is flawed: assert() lands in global namespace on Ubuntu 14.04, not std::
#include <functional>
#include <memory>
 
namespace mysql_harness {
 
// clang-format off
/* @brief: improved std::unique_ptr super-class
 *
 * std::unique_ptr has one problem: it's tedious to use with custom deleters.
 * This class is a convenience super-class, which makes working with unique_ptr
 * easier. Advantages:
 *
 *   1. Improved constructor for easier type declaration (deleter type is
 *      automatically resolved):
 *
 *        UniquePtr<Foo> foo(new_foo(), foo_deleter);
 *        UniquePtr<Foo> bar = std::move(foo);
 *
 *      instead of:
 *
 *        std::unique_ptr<Foo, decltype(foo_deleter)> foo(new_foo(), foo_deleter);
 *        std::unique_ptr<Foo, decltype(foo_deleter)> bar = std::move(foo);
 *
 *
 *
 *   2. UniquePtr constructor ALWAYS defines a custom deleter, which is a must
 *      if we're passing unique_ptr across DLL boundaries (the idea is to
 *      release the memory within the same DLL in which it was allocated).
 *      However, if all we need is a default deleter, it will implicitly define
 *      it for us:
 *
 *        UniquePtr<Foo> foo(new_foo()); // std::default_delete is default 2nd parameter
 *
 *      instead of:
 *
 *        std::unique_ptr<Foo, std::default_delete> foo(new_foo(), std::default_delete());
 *
 *
 *
 *   3. In debug builds, it offers a safety feature to warn the developer, if
 *      (s)he forgets about the custom deleter. See comments above release()
 *      method
 */
// clang-format on
template <typename T>
class UniquePtr : public std::unique_ptr<T, std::function<void(T *)>> {
 public:
  UniquePtr() = default;
 
  UniquePtr(T *ptr, std::function<void(T *)> deleter = std::default_delete<T>())
      : std::unique_ptr<T, std::function<void(T *)>>(ptr, deleter) {}
 
  UniquePtr(const UniquePtr<T> &) = delete;
  UniquePtr(UniquePtr<T> &&other)
      : std::unique_ptr<T, std::function<void(T *)>>::unique_ptr(
            std::move(other))
#ifndef NDEBUG
        ,
        get_deleter_called_(other.get_deleter_called_)
#endif
  {
  }
 
  UniquePtr &operator=(const UniquePtr<T> &) = delete;
  UniquePtr &operator=(UniquePtr<T> &&other) {
#ifndef NDEBUG
    get_deleter_called_ = other.get_deleter_called_;
#endif
    std::unique_ptr<T, std::function<void(T *)>>::operator=(std::move(other));
    return *this;
  }
 
// It is typical for developers never to worry about how they create and how
// they delete objects. Since we're using custom deleter, we want to make sure
// that the developer doesn't forget about that. We want to protect against a
// situation like so:
//
//   // some_place_1
//   UniquePtr<A> u_ptr(new A, my_custom_deleter(ptr); });
//
//   // some_place_2
//   A* raw_ptr = u_ptr.release();  // dev forgot that u_ptr also stores the
//                                  // deleter to erase this object
//   ...
//   ...
//   delete raw_ptr;  // KABOOOM!!!
//
// So the least we can do, is try to remind the developer if we notice the dev
// didn't bother fetching the deleter.
//
#ifndef NDEBUG
  T *release() {
    assert(get_deleter_called_);  // do you know what you're doing? (how are you
                                  // going to delete this object?)
    return std::unique_ptr<T, std::function<void(T *)>>::release();
  }
 
  typename UniquePtr<T>::deleter_type get_deleter() {
    get_deleter_called_ = true;  // dev didn't forget about the deleter, good!
    return std::unique_ptr<T, std::function<void(T *)>>::get_deleter();
  }
 
 private:
  bool get_deleter_called_ = false;
#endif
};
 
}  // namespace mysql_harness
#endif  // #ifndef MYSQL_HARNESS_UNIQUEPTR_INCLUDED