What are smart pointers?

• It is a pointer with automatic memory management

There are 3 smart pointer types in modern C++17.

• Unique pointer
• Shared pointer
• Weak pointer

The bread-and-butter ones are unique pointer and shared pointer.

Unique pointer

#include <memory>

// (preferred way)
std::unique_ptr<T> ptr = std::make_unique<T>();

// (another way)
std::unique_ptr<T> ptr(new T());

// when ptr goes out of scope,
// it will be deleted automatically

• Use make_unique<T> because it will raise an exception if some goes wrong in constructing T.

Usage

• It can be used just like a regular pointer.

ptr->method();

Shared pointer

• unique_ptr is called unique because there is only one pointer holding the address
• If it needs to be shared, use shared_ptr

There is a reference counter internally.

If one pointer is added to refer the same content, a reference counter increases. If each pointer goes out of scope, a reference counter decreases.

When there is no reference, its memory will be released.

Usage

• It's same as unique_ptr

std::shared_ptr<T> var = std::make_shared<T>();
// ok
std::shared_ptr<T> another = var;

Weak pointer

• It's like a shared_ptr but no reference counter is changed.

std::weak_ptr<T> var;
{
  std::shared_ptr<T> shared = std::make_shared<T>();

  // ok
  var = shared;
}
// `shared` will be deallocated
// var becomes undefined