C++ Type Erasure Type erasing type erasure with std::any
Example
This example uses C++14 and boost::any. In C++17 you can swap in std::any instead.
The syntax we end up with is:
const auto print =
make_any_method<void(std::ostream&)>([](auto&& p, std::ostream& t){ t << p << "\n"; });
super_any<decltype(print)> a = 7;
(a->*print)(std::cout);
which is almost optimal.
This example is based off of work by @dyp and @cpplearner as well as my own.
First we use a tag to pass around types:
template<class T>struct tag_t{constexpr tag_t(){};};
template<class T>constexpr tag_t<T> tag{};
This trait class gets the signature stored with an any_method:
This creates a function pointer type, and a factory for said function pointers, given an any_method:
template<class any_method>
using any_sig_from_method = typename any_method::signature;
template<class any_method, class Sig=any_sig_from_method<any_method>>
struct any_method_function;
template<class any_method, class R, class...Args>
struct any_method_function<any_method, R(Args...)>
{
template<class T>
using decorate = std::conditional_t< any_method::is_const, T const, T >;
using any = decorate<boost::any>;
using type = R(*)(any&, any_method const*, Args&&...);
template<class T>
type operator()( tag_t<T> )const{
return +[](any& self, any_method const* method, Args&&...args) {
return (*method)( boost::any_cast<decorate<T>&>(self), decltype(args)(args)... );
};
}
};
any_method_function::type is the type of a function pointer we will store alongside the instance. any_method_function::operator() takes a tag_t<T> and writes a custom instance of the any_method_function::type that assumes the any& is going to be a T.
We want to be able to type-erase more than one method at a time. So we bundle them up in a tuple, and write a helper wrapper to stick the tuple into static storage on a per-type basis and maintain a pointer to them.
template<class...any_methods>
using any_method_tuple = std::tuple< typename any_method_function<any_methods>::type... >;
template<class...any_methods, class T>
any_method_tuple<any_methods...> make_vtable( tag_t<T> ) {
return std::make_tuple(
any_method_function<any_methods>{}(tag<T>)...
);
}
template<class...methods>
struct any_methods {
private:
any_method_tuple<methods...> const* vtable = 0;
template<class T>
static any_method_tuple<methods...> const* get_vtable( tag_t<T> ) {
static const auto table = make_vtable<methods...>(tag<T>);
return &table;
}
public:
any_methods() = default;
template<class T>
any_methods( tag_t<T> ): vtable(get_vtable(tag<T>)) {}
any_methods& operator=(any_methods const&)=default;
template<class T>
void change_type( tag_t<T> ={} ) { vtable = get_vtable(tag<T>); }
template<class any_method>
auto get_invoker( tag_t<any_method> ={} ) const {
return std::get<typename any_method_function<any_method>::type>( *vtable );
}
};
We could specialize this for a cases where the vtable is small (for example, 1 item), and use direct pointers stored in-class in those cases for efficiency.
Now we start the super_any. I use super_any_t to make the declaration of super_any a bit easier.
template<class...methods>
struct super_any_t;
This searches the methods that the super any supports for SFINAE and better error messages:
template<class super_any, class method>
struct super_method_applies_helper : std::false_type {};
template<class M0, class...Methods, class method>
struct super_method_applies_helper<super_any_t<M0, Methods...>, method> :
std::integral_constant<bool, std::is_same<M0, method>{} || super_method_applies_helper<super_any_t<Methods...>, method>{}>
{};
template<class...methods, class method>
auto super_method_test( super_any_t<methods...> const&, tag_t<method> )
{
return std::integral_constant<bool, super_method_applies_helper< super_any_t<methods...>, method >{} && method::is_const >{};
}
template<class...methods, class method>
auto super_method_test( super_any_t<methods...>&, tag_t<method> )
{
return std::integral_constant<bool, super_method_applies_helper< super_any_t<methods...>, method >{} >{};
}
template<class super_any, class method>
struct super_method_applies:
decltype( super_method_test( std::declval<super_any>(), tag<method> ) )
{};
Next we create the any_method type. An any_method is a pseudo-method-pointer. We create it globally and constly using syntax like:
const auto print=make_any_method( [](auto&&self, auto&&os){ os << self; } );
or in C++17:
const any_method print=[](auto&&self, auto&&os){ os << self; };
Note that using a non-lambda can make things hairy, as we use the type for a lookup step. This can be fixed, but would make this example longer than it already is. So always initialize an any method from a lambda, or from a type parametarized on a lambda.
template<class Sig, bool const_method, class F>
struct any_method {
using signature=Sig;
enum{is_const=const_method};
private:
F f;
public:
template<class Any,
// SFINAE testing that one of the Anys's matches this type:
std::enable_if_t< super_method_applies< Any&&, any_method >{}, int>* =nullptr
>
friend auto operator->*( Any&& self, any_method const& m ) {
// we don't use the value of the any_method, because each any_method has
// a unique type (!) and we check that one of the auto*'s in the super_any
// already has a pointer to us. We then dispatch to the corresponding
// any_method_data...
return [&self, invoke = self.get_invoker(tag<any_method>), m](auto&&...args)->decltype(auto)
{
return invoke( decltype(self)(self), &m, decltype(args)(args)... );
};
}
any_method( F fin ):f(std::move(fin)) {}
template<class...Args>
decltype(auto) operator()(Args&&...args)const {
return f(std::forward<Args>(args)...);
}
};
A factory method, not needed in C++17 I believe:
template<class Sig, bool is_const=false, class F>
any_method<Sig, is_const, std::decay_t<F>>
make_any_method( F&& f ) {
return {std::forward<F>(f)};
}
This is the augmented any. It is both an any, and it carries around a bundle of type-erasure function pointers that change whenever the contained any does:
template<class... methods>
struct super_any_t:boost::any, any_methods<methods...> {
using vtable=any_methods<methods...>;
public:
template<class T,
std::enable_if_t< !std::is_base_of<super_any_t, std::decay_t<T>>{}, int> =0
>
super_any_t( T&& t ):
boost::any( std::forward<T>(t) )
{
using dT=std::decay_t<T>;
this->change_type( tag<dT> );
}
boost::any& as_any()&{return *this;}
boost::any&& as_any()&&{return std::move(*this);}
boost::any const& as_any()const&{return *this;}
super_any_t()=default;
super_any_t(super_any_t&& o):
boost::any( std::move( o.as_any() ) ),
vtable(o)
{}
super_any_t(super_any_t const& o):
boost::any( o.as_any() ),
vtable(o)
{}
template<class S,
std::enable_if_t< std::is_same<std::decay_t<S>, super_any_t>{}, int> =0
>
super_any_t( S&& o ):
boost::any( std::forward<S>(o).as_any() ),
vtable(o)
{}
super_any_t& operator=(super_any_t&&)=default;
super_any_t& operator=(super_any_t const&)=default;
template<class T,
std::enable_if_t< !std::is_same<std::decay_t<T>, super_any_t>{}, int>* =nullptr
>
super_any_t& operator=( T&& t ) {
((boost::any&)*this) = std::forward<T>(t);
using dT=std::decay_t<T>;
this->change_type( tag<dT> );
return *this;
}
};
Because we store the any_methods as const objects, this makes making a super_any a bit easier:
template<class...Ts>
using super_any = super_any_t< std::remove_cv_t<Ts>... >;
Test code:
const auto print = make_any_method<void(std::ostream&)>([](auto&& p, std::ostream& t){ t << p << "\n"; });
const auto wprint = make_any_method<void(std::wostream&)>([](auto&& p, std::wostream& os ){ os << p << L"\n"; });
int main()
{
super_any<decltype(print), decltype(wprint)> a = 7;
super_any<decltype(print), decltype(wprint)> a2 = 7;
(a->*print)(std::cout);
(a->*wprint)(std::wcout);
}
Originally posted here in a SO self question & answer (and people noted above helped with the implementation).
