C++ Type Erasure A move-only `std::function`
Example
std::function type erases down to a few operations. One of the things it requires is that the stored value be copyable.
This causes problems in a few contexts, like lambdas storing unique ptrs. If you are using the std::function in a context where copying doesn't matter, like a thread pool where you dispatch tasks to threads, this requirement can add overhead.
In particular, std::packaged_task<Sig> is a callable object that is move-only. You can store a std::packaged_task<R(Args...)> in a std::packaged_task<void(Args...)>, but that is a pretty heavy-weight and obscure way to create a move-only callable type-erasure class.
Thus the task. This demonstrates how you could write a simple std::function type. I omitted the copy constructor (which would involve adding a clone method to details::task_pimpl<...> as well).
template<class Sig>
struct task;
// putting it in a namespace allows us to specialize it nicely for void return value:
namespace details {
template<class R, class...Args>
struct task_pimpl {
virtual R invoke(Args&&...args) const = 0;
virtual ~task_pimpl() {};
virtual const std::type_info& target_type() const = 0;
};
// store an F. invoke(Args&&...) calls the f
template<class F, class R, class...Args>
struct task_pimpl_impl:task_pimpl<R,Args...> {
F f;
template<class Fin>
task_pimpl_impl( Fin&& fin ):f(std::forward<Fin>(fin)) {}
virtual R invoke(Args&&...args) const final override {
return f(std::forward<Args>(args)...);
}
virtual const std::type_info& target_type() const final override {
return typeid(F);
}
};
// the void version discards the return value of f:
template<class F, class...Args>
struct task_pimpl_impl<F,void,Args...>:task_pimpl<void,Args...> {
F f;
template<class Fin>
task_pimpl_impl( Fin&& fin ):f(std::forward<Fin>(fin)) {}
virtual void invoke(Args&&...args) const final override {
f(std::forward<Args>(args)...);
}
virtual const std::type_info& target_type() const final override {
return typeid(F);
}
};
};
template<class R, class...Args>
struct task<R(Args...)> {
// semi-regular:
task()=default;
task(task&&)=default;
// no copy
private:
// aliases to make some SFINAE code below less ugly:
template<class F>
using call_r = std::result_of_t<F const&(Args...)>;
template<class F>
using is_task = std::is_same<std::decay_t<F>, task>;
public:
// can be constructed from a callable F
template<class F,
// that can be invoked with Args... and converted-to-R:
class= decltype( (R)(std::declval<call_r<F>>()) ),
// and is not this same type:
std::enable_if_t<!is_task<F>{}, int>* = nullptr
>
task(F&& f):
m_pImpl( make_pimpl(std::forward<F>(f)) )
{}
// the meat: the call operator
R operator()(Args... args)const {
return m_pImpl->invoke( std::forward<Args>(args)... );
}
explicit operator bool() const {
return (bool)m_pImpl;
}
void swap( task& o ) {
std::swap( m_pImpl, o.m_pImpl );
}
template<class F>
void assign( F&& f ) {
m_pImpl = make_pimpl(std::forward<F>(f));
}
// Part of the std::function interface:
const std::type_info& target_type() const {
if (!*this) return typeid(void);
return m_pImpl->target_type();
}
template< class T >
T* target() {
return target_impl<T>();
}
template< class T >
const T* target() const {
return target_impl<T>();
}
// compare with nullptr :
friend bool operator==( std::nullptr_t, task const& self ) { return !self; }
friend bool operator==( task const& self, std::nullptr_t ) { return !self; }
friend bool operator!=( std::nullptr_t, task const& self ) { return !!self; }
friend bool operator!=( task const& self, std::nullptr_t ) { return !!self; }
private:
template<class T>
using pimpl_t = details::task_pimpl_impl<T, R, Args...>;
template<class F>
static auto make_pimpl( F&& f ) {
using dF=std::decay_t<F>;
using pImpl_t = pimpl_t<dF>;
return std::make_unique<pImpl_t>(std::forward<F>(f));
}
std::unique_ptr<details::task_pimpl<R,Args...>> m_pImpl;
template< class T >
T* target_impl() const {
return dynamic_cast<pimpl_t<T>*>(m_pImpl.get());
}
};
To make this library-worthy, you'd want to add in a small buffer optimization, so it does not store every callable on the heap.
Adding SBO would require a non-default task(task&&), some std::aligned_storage_t within the class, a m_pImpl unique_ptr with a deleter that can be set to destroy-only (and not return the memory to the heap), and a emplace_move_to( void* ) = 0 in the task_pimpl.
live example of the above code (with no SBO).
