Opaque owning pointer

owning.hpp

#include <memory>

template <class T, class Deleter = std::default_delete<T>>
class owning_ptr {
  private:
    T* ptr;
    Deleter deleter;
  
  explicit owning_ptr(T* rawPtr, Deleter d = {}) : ptr{rawPtr}, deleter{d} {}
  
  const T* as_immutable() const { return ptr; }
  
  T* as_mutable() { return ptr; }
  
  ~owning_ptr() {
    deleter(ptr);
    ptr = nullptr;
  }
};