Unique_Ptr in C++
This article and the examples are from the Microsoft C++ Documentation.
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 C++ Standard Library 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.
Examples
Example 1
The following example shows how to create unique_ptr instances and pass them between functions. These are usually the 3 ways to create unique_ptr instances. It shows that the unique_ptr can be moved, but not copied. “Moving” transfers ownership to a new unique_ptr and resets the old unique_ptr.
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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 MakeSong() {
// 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");
}
Example 2
The following example shows how to create unique_ptr instances and use them in a vector.
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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.
Example 3
The following example shows how to initialize a unique_ptr that is a class member.
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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();
}
}
Example 4
You can use make_unique to create a unique_ptr to an array, but you cannot use make_unique to initialize the array elements.
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// 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;
}