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.
.png "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.
Example
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.