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How to: Create and Use unique_ptr Instances

A unique_ptr does not share its pointer. It cannot be copied to another unique_ptr, passed by value to a function, or used in any Standard Template Library (STL) algorithm that requires copies to be made. A unique_ptr can only be moved. This means that the ownership of the memory resource is transferred to another unique_ptr and the original unique_ptr no longer owns it. We recommend that you restrict an object to one owner, because multiple ownership adds complexity to the program logic. Therefore, when you need a smart pointer for a plain C++ object, use unique_ptr, and when you construct a unique_ptr, use the make_unique helper function.

The following diagram illustrates the transfer of ownership between two unique_ptr instances.

Moving the ownership of a unique_ptr

unique_ptr is defined in the <memory> header in the STL. It is exactly is efficient as a raw pointer and can be used in STL containers. The addition of unique_ptr instances to STL containers is efficient because the move constructor of the unique_ptr eliminates the need for a copy operation.

The following example shows how to create unique_ptr instances and pass them between functions.

unique_ptr<Song> SongFactory(const std::wstring& artist, const std::wstring& title)
{
    // Implicit move operation into the variable that stores the result. 
    return make_unique<Song>(artist, title);
}

void MakeSongs()
{
    // Create a new unique_ptr with a new object.
    auto song = make_unique<Song>(L"Mr. Children", L"Namonaki Uta");

    // Use the unique_ptr.
    vector<wstring> titles = { song->title };

    // Move raw pointer from one unique_ptr to another.
    unique_ptr<Song> song2 = std::move(song);

    // Obtain unique_ptr from function that returns by value.
    auto song3 = SongFactory(L"Michael Jackson", L"Beat It");
}

These examples demonstrate this basic characteristic of unique_ptr: it can be moved, but not copied. "Moving" transfers ownership to a new unique_ptr and resets the old unique_ptr.

The following example shows how to create unique_ptr instances and use them in a vector.

void SongVector()
{
    vector<unique_ptr<Song>> songs;

    // Create a few new unique_ptr<Song> instances 
    // and add them to vector using implicit move semantics.
    songs.push_back(make_unique<Song>(L"B'z", L"Juice"));
    songs.push_back(make_unique<Song>(L"Namie Amuro", L"Funky Town"));
    songs.push_back(make_unique<Song>(L"Kome Kome Club", L"Kimi ga Iru Dake de"));
    songs.push_back(make_unique<Song>(L"Ayumi Hamasaki", L"Poker Face"));

    // Pass by const reference when possible to avoid copying. 
    for (const auto& song : songs)
    {
        wcout << L"Artist: " << song->artist << L"   Title: " << song->title << endl; 
    }    
}

In the range for loop, notice that the unique_ptr is passed by reference. If you try to pass by value here, the compiler will throw an error because the unique_ptr copy constructor is deleted.

The following example shows how to initialize a unique_ptr that is a class member.


class MyClass
{
private:
    // MyClass owns the unique_ptr.
    unique_ptr<ClassFactory> factory;
public:

    // Initialize by using make_unique with ClassFactory default constructor.
    MyClass() : factory ( make_unique<ClassFactory>())
    {
    }

    void MakeClass()
    {
        factory->DoSomething();
    }
};

You can use make_unique to create a unique_ptr to an array, but you cannot use make_unique to initialize the array elements.

	// Create a unique_ptr to an array of 5 integers.
	auto p = make_unique<int[]>(5);

	// Initialize the array. 
	for (int i = 0; i < 5; ++i)
	{
		p[i] = i;
		wcout << p[i] << endl;
	}

For more examples, see make_unique.

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