C++: What is the difference between std::unique_ptr and std::shared_ptr?

unique_ptr — exclusive ownership. shared_ptr — shared ownership.

std::unique_ptr:

• Exclusive ownership.
• Cannot be copied, only moved.
• No control block (lightweight).
• No reference counting.

1auto ptr1 = std::make_unique<int>(10);
2// auto ptr2 = ptr1;  // Error! Cannot copy
3auto ptr2 = std::move(ptr1);  // OK — move

std::shared_ptr:

• Shared ownership.
• Can be copied.
• Reference counting.
• Control block overhead.

1auto ptr1 = std::make_shared<int>(10);
2auto ptr2 = ptr1;  // OK — share ownership
3std::cout << ptr1.use_count();  // 2

When to use:

• unique_ptr — default choice, exclusive ownership.
• shared_ptr — multiple owners needed.
• weak_ptr — break circular references.