PImpl

PIMPL, short for Pointer to IMPLementation, is a C++ design idiom used to separate a class’s interface from its implementation. By moving private data members and implementation details into a separate class and keeping a pointer to that class in the public interface, the header file exposes only the minimal interface while hiding the rest of the implementation.

In a typical PIMPL setup, the public class forward declares an inner or separate Impl class and stores a pointer, often a smart pointer such as std::unique_ptr, to an Impl instance. The Impl class is defined in the source file, where the private data and helper methods reside. Public member functions are implemented to delegate work to the Impl object. This arrangement reduces header dependencies, since changes to the implementation do not force recompilation of files including the public header.

Advantages include improved encapsulation, reduced compile times, and the potential for better binary compatibility across library versions. It can also help with stable ABIs, since changes to private members do not affect the public interface. Drawbacks include added indirection, increased boilerplate, and potential performance costs. Copying and moving objects with PIMPL requires explicit rule-of-five handling and careful management of the Impl resource, especially when not using move-only semantics.

PIMPL is most beneficial for large, stable interfaces in libraries where compile-time dependencies and binary compatibility are important. It is less suitable for small classes or performance-critical code where the extra indirection and boilerplate are undesirable.