unique_ptr不共享其指针。 它不能复制到另一个 unique_ptr函数，由值传递给函数，也不能在任何需要复制副本的 C++ 标准库算法中使用。 只能移动 unique_ptr。 这意味着，内存资源所有权将转移到另一 unique_ptr，并且原始 unique_ptr 不再拥有此资源。 建议将对象限制为一个所有者，因为多个所有权增加了复杂性。 当您需要为一个纯 C++ 对象使用智能指针时，请使用unique_ptr；当您构造unique_ptr时，请使用make_unique辅助函数。
下图演示了两个 unique_ptr 实例之间的所有权转换。
unique_ptr 在 C++ 标准库的 <memory> 标头中定义。 它与原始指针一样高效，可在 C++ 标准库容器中使用。 将 unique_ptr 实例添加到 C++ 标准库容器很有效，因为通过 unique_ptr 的移动构造函数，不再需要进行复制操作。
若要使用 unique_ptr 和 make_unique，请包含 <memory> 标头。 以下示例每个编译并作为独立程序运行。
示例 1
以下示例演示如何创建 unique_ptr 实例并在函数之间传递这些实例。 通过按值返回 unique_ptr 将所有权转让给调用方。 将 unique_ptr 按值传递给函数会将所有权传递给调用方。
// Compile with: cl /EHsc /std:c++17
#include <iostream>
#include <memory>
#include <string>
struct Song {
std::string artist;
std::string title;
Song(const std::string& a, const std::string& t) : artist(a), title(t) {
std::cout << "Created: " << title << "\n";
}
~Song() { std::cout << "Destroyed: " << title << "\n"; }
};
// Returning a unique_ptr by value transfers ownership to the caller.
std::unique_ptr<Song> SongFactory(const std::string& artist, const std::string& title) {
return std::make_unique<Song>(artist, title);
}
// Passing a unique_ptr by value transfers ownership to the function.
// The Song is automatically destroyed when the function exits.
void SingSong(std::unique_ptr<Song> song) {
std::cout << "Singing: " << song->title << " by " << song->artist << "\n";
}
int main() {
// Create a new unique_ptr with a new object.
auto song = std::make_unique<Song>("Mr. Children", "Namonaki Uta");
std::cout << "song points to: " << song->title << "\n";
// Move ownership from one unique_ptr to another.
std::unique_ptr<Song> song2 = std::move(song);
std::cout << "After move, song is " << (song ? "not null" : "null") << "\n";
std::cout << "song2 points to: " << song2->title << "\n";
// Obtain unique_ptr from a factory function that returns by value.
auto song3 = SongFactory("Michael Jackson", "Beat It");
// Transfer ownership to a function.
SingSong(std::move(song3));
std::cout << "After SingSong, song3 is " << (song3 ? "not null" : "null") << "\n";
}
Created: Namonaki Uta
song points to: Namonaki Uta
After move, song is null
song2 points to: Namonaki Uta
Created: Beat It
Singing: Beat It by Michael Jackson
Destroyed: Beat It
After SingSong, song3 is null
Destroyed: Namonaki Uta
这些示例说明了 unique_ptr 的基本特征：可移动，但不可复制。 “移动”将所有权转移到新 unique_ptr 并重置旧 unique_ptr。
示例 2
以下示例演示如何创建 unique_ptr 实例并在向量中使用这些实例。
// Compile with: cl /EHsc /std:c++17
#include <iostream>
#include <memory>
#include <string>
#include <vector>
struct Song {
std::string artist;
std::string title;
Song(const std::string& a, const std::string& t) : artist(a), title(t) {}
};
int main() {
std::vector<std::unique_ptr<Song>> songs;
// Create unique_ptr<Song> instances and add them to the vector
// using implicit move semantics.
songs.push_back(std::make_unique<Song>("B'z", "Juice"));
songs.push_back(std::make_unique<Song>("Namie Amuro", "Funky Town"));
songs.push_back(std::make_unique<Song>("Kome Kome Club", "Kimi ga Iru Dake de"));
songs.push_back(std::make_unique<Song>("Ayumi Hamasaki", "Poker Face"));
// Pass by const reference to avoid copying.
// Passing by value causes a compile error because
// the unique_ptr copy constructor is deleted.
for (const auto& song : songs) {
std::cout << "Artist: " << song->artist
<< " Title: " << song->title << "\n";
}
// The unique_ptr instances in the vector are automatically destroyed when the vector goes out of scope at the end of main()
}
Artist: B'z Title: Juice
Artist: Namie Amuro Title: Funky Town
Artist: Kome Kome Club Title: Kimi ga Iru Dake de
Artist: Ayumi Hamasaki Title: Poker Face
Destroyed: Juice
Destroyed: Funky Town
Destroyed: Kimi ga Iru Dake de
Destroyed: Poker Face
在 range for 循环中，注意 unique_ptr 通过引用来传递。 如果尝试在此处传递值，编译器会报告错误，因为 unique_ptr 复制构造函数已删除。
示例 3
以下示例演示如何初始化类成员 unique_ptr。
// Compile with: cl /EHsc /std:c++17
#include <iostream>
#include <memory>
class Engine {
public:
Engine() { std::cout << "Engine created\n"; }
~Engine() { std::cout << "Engine destroyed\n"; }
void Run() { std::cout << "Engine running\n"; }
};
class Car {
private:
// Car owns the unique_ptr.
std::unique_ptr<Engine> engine;
public:
// Initialize by using make_unique in the member initializer list.
Car() : engine(std::make_unique<Engine>()) {}
void Start() {
engine->Run();
}
};
int main() {
Car car;
car.Start();
}
Engine created
Engine running
Engine destroyed
示例 4
可以使用 make_unique 创建 unique_ptr 数组。
make_unique<int[]>(5) 创建一个值初始化为零的五个元素数组。 不能将 make_unique单个元素值传递到其中，因此在创建后对其进行分配。
// Compile with: cl /EHsc /std:c++17
#include <iostream>
#include <memory>
int main() {
// Create a unique_ptr to an array of 5 integers.
// The elements are value-initialized to 0.
auto p = std::make_unique<int[]>(5);
// Assign values to the array elements.
for (int i = 0; i < 5; ++i) {
p[i] = i;
std::cout << p[i] << "\n";
}
// The array is automatically deleted when p goes out of scope.
}
0
1
2
3
4
有关更多示例，请参阅 make_unique。