# My unique_ptr implementation

This is my simple unique_ptr implementation. Anything that could be improved upon or should be added?

#include <algorithm>

template<typename T>
class unique_ptr
{
private:
T * ptr_resource = nullptr;
public:
// Safely constructs resource. Operator new is called by the user. Once constructed the unique_ptr will own the resource.
// std::move is used because it is used to indicate that an object may be moved from other resource.
explicit unique_ptr(T* raw_resource) noexcept : ptr_resource(std::move(raw_resource)) {}
unique_ptr(std::nullptr_t) : ptr_resource(nullptr) {}

// destroys the resource when object goes out of scope
~unique_ptr() noexcept
{
delete ptr_resource;
}
// Disables the copy/ctor and copy assignment operator. We cannot have two copies exist or it'll bypass the RAII concept.
unique_ptr(const unique_ptr<T>&) noexcept = delete;
unique_ptr& operator = (const unique_ptr&) noexcept = delete;
public:
// releases the ownership of the resource. The user is now responsible for memory clean-up.
T* release() noexcept
{
T* resource_ptr = this->ptr_resource;
this->ptr_resource = nullptr;

return resource_ptr;
}
// returns a pointer to the resource
T* get() const noexcept
{
return ptr_resource;
}
// swaps the resources
void swap(unique_ptr<T>& resource_ptr) noexcept
{
std::swap(ptr_resource, resource_ptr.ptr_resource);
}
// replaces the resource. the old one is destroyed and a new one will take it's place.
void reset(T* resource_ptr) noexcept(false)
{
// ensure a invalid resource is not passed or program will be terminated
if (resource_ptr == nullptr)
throw std::invalid_argument("An invalid pointer was passed, resources will not be swapped");

delete ptr_resource;

ptr_resource = nullptr;

std::swap(ptr_resource, resource_ptr);
}
public:
// operators
T* operator->() const noexcept
{
return this->ptr_resource;
}
T& operator*() const noexcept
{
return *this->ptr_resource;
}
// May be used to check for nullptr
};


• Let me first assume that your unique_ptr is supposed to be movable. Then, any basic test case whould have unrevealed this:

unique_ptr<int> ptr1(new int());
unique_ptr<int> ptr2 = std::move(ptr1); // Fails to compile


Recall that = delete-ing special member functions means user-declaring them. And user-declared copy and copy assignment constructors prevent compiler-generated move (assignment) constructors! You have to manually define them. This case is by the way covered by the rule of five/C.21 Core Guidelines, and also have a look at the table posted in this SO answer for an overview of compiler-generated/-deleted/not-declared special member functions.

• Is the non-availability of an implicit conversion to bool intended? Checking if a (smart) pointer is in an empty/null state is so common in ordinary control flow statements that clients will expect this to compile:

unique_ptr<SomeType> ptr = ...;

if (ptr) ... // currently fails to compile


But not that this might be debatable. Implicit conversions can cause a lot of pain, so if you intend to not allow them for the sake of a more explicit

if (ptr == nullptr) ...


that's a design decision. But one that should be documented in a comment at the top of the class.

• Except the non-explicit-ness of the second constructor (thanks to @Deduplicator for pointing that out) taking a std::nullptr_t, it is superfluous. You can construct an empty unique_ptr by

unique_ptr<SomeType> empty{nullptr};


which simply invokes the first constructor taking a T* argument. I would remove the second constructor.

• ... and add a default constructor that initializes ptr_resource to nullptr, as

unique_tr<SomeType> empty;


might be a way of constructing an empty smart pointer that users would expect to compile.

• Move-constructing the ptr_resource in the constructor initializer by ptr_resource(std::move(raw_resource)) doesn't make much sense. Just copy the pointer instead. The comment // std::move is used because it is used to indicate that an object may be moved from other resource. is rather confusing, because T* raw_resource is already a pointer, and hence a handle to a resource, not the resource itself.

• The release member function can be implemented more conveniently as

T* release() noexcept
{
return std::exchange(ptr_resource, nullptr);
}

• I wouln't let the reset member function throw when the input is a nullptr. Why shouldn't it be allowed to reset a unique_ptr with a nullptr, turning it back into an empty state?

• The only facilities you use from the standard library are std::move and std::swap. Those are in <utility>, so you don't need to include <algorithm>, which is probably much heavier in terms of compile times.

• I would omit the this-> prefix, it's unnecessarily verbose. But that might be a matter of taste.

• Have you considered custom deleters? This makes the class template more reusable in scenarios other than pointers to heap resources, e.g. closing a file upon destruction etc.