The PImpl Idiom and the std:indirect Type in C++26
Discover how the PImpl idiom works in C++ programming and the conveniences brought by C++26.
1What is the PImpl Idiom?
The PImpl (Pointer to Implementation) idiom is a programming technique used to hide the implementation details of a class. This technique separates the interface from the implementation, minimizing compile-time dependencies.
2Implementation with Raw Pointers
The PImpl idiom can be implemented using raw pointers. In this approach, the 'Rule of Five' is followed, and all five special member functions of the class are defined. These functions are essential for resource management.
3Widget Example
The Widget class represents a UI element and has a clickable structure. Each click increments a counter, and these details are hidden behind an implementation class. The Widget class accesses these details through a pointer.
4Constness Issue
Constness does not propagate from Widget to Impl in the Widget class. This means that even within a const method, the state of the Impl object can be changed. This situation can lead to unexpected results.
5Move Semantics Issue
When move semantics are used, the pimpl_ pointer of the moved Widget object becomes null. In this case, any function called using the pointer results in undefined behavior (UB).
6Solution with std::unique_ptr
The PImpl idiom can be implemented more simply using std::unique_ptr from C++11. This automates resource management and provides a safer solution compared to using raw pointers.
7The New Widget Class
When std::unique_ptr is used, the copy constructor and copy assignment operator of the Widget class are explicitly defined. Other special member functions are defaulted by the compiler.
8Conveniences with C++26
The std::indirect type introduced with C++26 will further simplify the implementation of the PImpl idiom. This new type will make pointer management more efficient and the code more readable.
Original article: Hacker News — C++26'da PImpl İdiomu ve std:indirect Türü