More C++ Idioms/Type Erasure

Type Erasure

Intent

To provide a type-neutral container that interfaces a variety of concrete types.

Also Known As

"Variant" (not to be confused with std::variant). This technique is used inside std::any and std::function.

Motivation

It is often useful to have a variable which can contain more than one type. Type Erasure is a technique to represent a variety of concrete types through a single generic interface.

Implementation and Example

Type Erasure is achieved in C++ by encapsulating a concrete implementation in a generic wrapper and providing virtual accessor methods to the concrete implementation via a generic interface.

The key components in this example interface are var, inner_base and inner classes:

struct var{
   struct inner_base{
      using ptr = std::unique_ptr<inner_base>;
   };
   template <typename _Ty> struct inner : inner_base{};
private:
   typename inner_base::ptr _inner;
};

The var class holds a pointer to the inner_base class. Concrete implementations on inner (such as inner<int> or inner<std::string>) inherit from inner_base. The var representation will access the concrete implementations through the generic inner_base interface. To hold arbitrary types of data a little more scaffolding is needed:

struct var{
   template <typename _Ty> var(_Ty src) : _inner(new inner<_Ty>(std::forward<_Ty>(src))) {} //construct an internal concrete type accessible through inner_base
   struct inner_base{
      using ptr = std::unique_ptr<inner_base>;
   };
   template <typename _Ty> struct inner : inner_base{
      inner(_Ty newval) : _value(newval) {}
   private:
      _Ty _value;
   };
private:
   typename inner_base::ptr _inner;
};

The utility of an erased type is to assign multiple typed values to it so an assignment operator achieves just that:

struct var{
   template <typename _Ty> var& operator = (_Ty src) {
      _inner = std::make_unique<inner<_Ty>>(std::forward<_Ty>(src));
      return *this;
   }
   struct inner_base{
      using ptr = std::unique_ptr<inner_base>;
   };
   template <typename _Ty> struct inner : inner_base{
      inner(_Ty newval) : _value(newval) {}
   private:
      _Ty _value;
   };
private:
   typename inner_base::ptr _inner;
};

Creating an erased type and assigning it various values isn't of much use unless you can interrogate it. One useful method is to query for the underlying type info:

struct var{
   const std::type_info& Type() const { return _inner->Type(); }
   struct inner_base{
      using ptr = std::unique_ptr<inner_base>;
      virtual const std::type_info& Type() const = 0;
   };
   template <typename _Ty> struct inner : inner_base{
      virtual const std::type_info& Type() const override { return typeid(_Ty); }
   };
private:
   typename inner_base::ptr _inner;
};

Here the var class forwards calls of Type() to it's inner_base interface which is overridden by the concrete inner<_Ty> subclass which ultimately returns the underlying type. This technique of forwarding accessor methods to a virtual interface which is overridden by concrete implementations is expanded for a fully useful generic type.

Complete Implementation

struct var {
   var() : _inner(new inner<int>(0)){} //default construct to an integer

   var(const var& src) : _inner(src._inner->clone()) {} //copy constructor calls clone method of concrete type

   template <typename _Ty> var(_Ty src) : _inner(new inner<_Ty>(std::forward<_Ty>(src))) {}

   template <typename _Ty> var& operator = (_Ty src) { //assign to a concrete type
      _inner = std::make_unique<inner<_Ty>>(std::forward<_Ty>(src));
      return *this;
   }

   var& operator=(const var& src) { //assign to another var type
      var oTmp(src);
      std::swap(oTmp._inner, this->_inner);
      return *this;
   }

   //interrogate the underlying type through the inner_base interface
   const std::type_info& Type() const { return _inner->Type(); }
   bool IsPOD() const { return _inner->IsPOD(); }
   size_t Size() const { return _inner->Size(); }

   //cast the underlying type at run-time
   template <typename _Ty> _Ty& cast() {
      return *dynamic_cast<inner<_Ty>&>(*_inner);
   }

   template <typename _Ty> const _Ty& cast() const {
      return *dynamic_cast<inner<_Ty>&>(*_inner);
   }

   struct inner_base {
      using Pointer = std::unique_ptr < inner_base > ;
      virtual ~inner_base() {}
      virtual inner_base * clone() const = 0;
      virtual const std::type_info& Type() const = 0;
      virtual bool IsPOD() const = 0;
      virtual size_t Size() const = 0;
   };

   template <typename _Ty> struct inner : inner_base {
      inner(_Ty newval) : _value(std::move(newval)) {}
      virtual inner_base * clone() const override { return new inner(_value); }
      virtual const std::type_info& Type() const override { return typeid(_Ty); }
      _Ty & operator * () { return _value; }
      const _Ty & operator * () const { return _value; }
      virtual bool IsPOD() const { return std::is_pod<_Ty>::value; }
      virtual size_t Size() const { return sizeof(_Ty); }
   private:
      _Ty _value;
   };

   inner_base::Pointer _inner;
};

//this is a specialization of an erased std::wstring
template <>
struct var::inner<std::wstring> : var::inner_base{
   inner(std::wstring newval) : _value(std::move(newval)) {}
   virtual inner_base * clone() const override { return new inner(_value); }
   virtual const std::type_info& Type() const override { return typeid(std::wstring); }
   std::wstring & operator * () { return _value; }
   const std::wstring & operator * () const { return _value; }
   virtual bool IsPOD() const { return false; }
   virtual size_t Size() const { return _value.size(); }
   private:
   std::wstring _value;
};

Example Implementation from Sean Parent talk

template<typename T>
void draw(const T& x, std::ostream& out, size_t position) {
	out << std::string(position, ' ') << x << std::endl;
}
class object_t {
public:
	template<typename T>
	object_t(T x) : self_(std::make_shared<model<T>>(std::move(x))) {}
	friend void draw(const object_t& x, std::ostream& out, size_t position) {
		x.self_->draw_(out, position);
	}
private:
	struct concept_t {
		virtual ~concept_t() = default;
		virtual void draw_(std::ostream&, size_t) const = 0;
	};
	template<typename T> 
	struct model final : concept_t {
		model(T x) : data_(std::move(x)) {}
		void draw_(std::ostream& out, size_t position) const override {
			draw(data_, out, position);
		}
		T data_;
	};
	std::shared_ptr<const concept_t>self_;
};