Propagate constness to data pointed by member variables

Question

It is often quite confusing to C++ newcomers that const member functions are allowed to call non-const methods on objects referenced by the class (either by pointer or refer

Answer 1

One approach is to just not use the pointer directly except through two accessor functions.

class SomeClass
{
  private:
    class SomeClassImpl;
    SomeClassImpl * impl_; // PImpl idiom - don't use me directly!

    SomeClassImpl * mutable_impl() { return impl_; }
    const SomeClassImpl * impl() const { return impl_; }

  public:    

    void const_method() const
    {
      //Can't use mutable_impl here.
      impl()->const_method();
    }
    void non_const_method() const
    {
      //Here I can use mutable_impl
      mutable_impl()->non_const_method();
    }
};