smart_ptr
smart_ptr is my own implementation of C++ smart pointers. It implements the smart pointers part (§20.7) of ISO C++ 2011 with some useful new features added (like make_unique) and some features removed (like auto_ptr and custom allocator for shared_ptr).
Motivation
Smart pointers are a distinctive feature of modern C++. They enable automatic and exception-safe object lifetime management. Implementing smart pointers is a great opportunity to deepen my understanding of smart pointers themselves and dynamic memory management in gerenal. The implementation also involves an understanding of templates, copy control and operator overloading, all important concepts of C++.
Features
smart_ptr implements the smart pointers part (§20.7) of ISO C++ 2011 with a few exceptions. Custom deleter, various non-member helper funcitons, enable_shared_from_this class, owner_less class, as well as the std::hash class template specialization are supported. However, custom allocator for shared_ptr is not supoorted. I also do few checkings for template argument requirements as they are too tedious for educational purposes. For example, I do not explicitly check whether two pointer types are convertible, or whether a custom deleter type is copy-constructible. Conforming to these implicit requirements is left to the users.
It includes the following smart pointers and helper classes:
| Smart Pointer | Description |
|---|---|
| unique_ptr | smart pointer with exclusive object ownership semantics |
| shared_ptr | smart pointer with shared object ownership semantics |
| weak_ptr | weak reference to an object managed by shared_ptr |
| Helper class | Description |
|---|---|
| bad_weak_ptr | exception thrown when accessing an expired weak_ptr |
| default_delete | default deleter used by smart pointers |
| enable_shared_from_this | allows an object to create a shared_ptr referring to itself |
| owner_less | provides mixed-type owner-based ordering of shared and weak pointers |
A list of the extra features and the removed ones are given below. Notes regarding the status of those features in more recent C++ versions are given in brackets.
Extra features
- make_unique (added in C++14)
- array type support for shared_ptr (added in C++17)
- reinterpret_pointer_cast for shared_ptr (added in C++17)
- operator« for unique_ptr (added in C++20)
Removed features
- auto_ptr (deprecated in C++11, removed in C++17)
- custom allocator for shared_ptr and allocate_shared
- specialized atomic operations for shared_ptr (deprecated in C++20)
Requirement
To include, simply include “smart_ptr.hpp”, C++11 required. All names are defined in the smart_ptr namespace except for _control_block_base and _control_block, which are defined in the smart_ptr::detail namespace.
To run the demo, run Makefile, pthread support required.
Implementation
Empty base optimization to store stateless deleter with zero size fee
A unique_ptr (as well as the control block) must always store its deleter. I can store a pointer to deleter in the unique_ptr at the cost of 1 pointer size of additional space. Alterantively, I can store the deleter directly in the unique_ptr, but since even an empty class will occupy space (see more here), I also pay additional space to store it. However, if the deleter is a stateless class type (like std::default_delete, or a lambda expression with empty capture list), then the unique_ptr can make use of empty base optimization (EBO) so that the deleter does not use any additional space. In my implementation, I used std::tuple, which is defined with recursivelly inheritance and does EBO automatically for me, to store the pointer to object and the deleter as a pair.
Indirect management through control block
shared_ptr and weak_ptr do not directly manage the object. They do it indirectly through the control block. The control block will stay alive until no associated shared_ptr/weak_ptr lives. In this way, when we try to use a weak_ptr which points to an already destroyed object, we can learn from the control block that this weak_ptr has been expired and the object no longer exists.
Type erasure of deleter in shared_ptr/weak_ptr
The template class of shared_ptr and weak_ptr has only one type parameter, which is the element type, and have no direct information about the type of the deleter. This is to ease the swap and assignment between different shared_ptrs/weak_ptrs (They may share the same element type, but have different deleter types. In that case, type conversion is difficult to handle.). However, this type information is required for deleter to function properly. To overcome this difficulty, I need to erase the type of the deleter (since they are stored in the control block, this is equivalent to erasing the type of the control block) in shared_ptr/weak_ptr. I accomplish it by relying on the runtime polymorphic behavior of a type with virtual functions. I first define a base class _control_block_base that defines the public interface. I then define a derived class control_block that contains all the neccessay type information and does the real stuff. The template class of shared_ptr/weak_ptr only stores a pointer to _control_block_base. When shared_ptr/weak_ptr is constructed, the constructors are supplied with the correct type information of the deleter, which can be used to initilize a control_block object. The pointer itself is of type _control_block_base*, but it points to an object of type control_block. Relying on the runtime polymorphic behavior of C++, all control block operations are called through the _control_block_base interface, but actually done in the control_block object.
class _control_block_base {
... // defines the interface
};
template<typename T, typename D = default_delete<T>>
class control_block : public _control_block_base {
... // actual implementation
};
Note
- Since the access to ISO/IEC documents are not public, I refered to N3337, which is the same as the C++11 standard but with a few typographical corrections.