In the header file:

// widget.h

#include <memory>  // std::unique_ptr
#include <experimental/propagate_const>

class Widget
{
    public:
        Widget();
        ~Widget();
        void DoSomething();

    private:
        // the pImpl idiom is named after the typical variable name used
        // ie, pImpl:
        struct Impl;                    // forward declaration
        std::experimental::propagate_const<std::unique_ptr< Impl >> pImpl;  // ptr to actual implementation
};

In the implementation file:

// widget.cpp

#include "widget.h"
#include "reallycomplextype.h" // no need to include this header inside widget.h

struct Widget::Impl
{
    // the attributes needed from Widget go here
    ReallyComplexType rct;
};

Widget::Widget() :
    pImpl(std::make_unique<Impl>())
{}

Widget::~Widget() = default;

void Widget::DoSomething()
{
    // do the stuff here with pImpl
}

The pImpl contains the Widget state (or some/most of it). Instead of the Widget description of state being exposed in the header file, it can be only exposed within the implementation.

pImpl stands for "pointer to implementation". The "real" implementation of Widget is in the pImpl.

Danger: Note that for this to work with unique_ptr, ~Widget() must be implemented at a point in a file where the Impl is fully visible. You can =default it there, but if =default where Impl is undefined, the program may easily become ill-formed, no diagnostic required.

Remarks

The pimpl idiom (pointer to implementation, sometimes referred to as opaque pointer or cheshire cat technique), reduces the compilation times of a class by moving all its private data members into a struct defined in the .cpp file.

The class owns a pointer to the implementation. This way, it can be forward declared, so that the header file does not need to #include classes that are used in private member variables.

When using the pimpl idiom, changing a private data member does not require recompiling classes that depend on it.