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// Copyright (C) 2021-2023 Free Software Foundation, Inc.
// This file is part of GCC.
// GCC is free software; you can redistribute it and/or modify it under
// the terms of the GNU General Public License as published by the Free
// Software Foundation; either version 3, or (at your option) any later
// version.
// GCC is distributed in the hope that it will be useful, but WITHOUT ANY
// WARRANTY; without even the implied warranty of MERCHANTABILITY or
// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
// for more details.
// You should have received a copy of the GNU General Public License
// along with GCC; see the file COPYING3. If not see
// <http://www.gnu.org/licenses/>.
// An improved implementation of the inline visitor.
// Original idea from https://members.accu.org/index.php/articles/2021
#ifndef RUST_INLINE_VISITOR
#define RUST_INLINE_VISITOR
namespace Rust {
// Wrapper for the target Visitor we're matching against.
// Consumes the final nullptr of the _args linked list.
template <typename TargetVisitor> struct EmptyVisitor : TargetVisitor
{
EmptyVisitor (std::nullptr_t ptr) {}
using TargetVisitor::visit;
};
// Wrapper for a (possibly incomplete) Visitor.
template <typename BaseVisitor, typename Args> struct VisitorWrapper
{
// Lambdas are stored in _args as a linked list and passed to the actual
// visitor when end_visitor() is called.
Args _args;
// The actual visitor being created.
// Each visitor inherits from the last one and implements one more visit().
template <typename T, typename F> struct Visitor : BaseVisitor
{
F _f;
Visitor (std::pair<F, Args> &&args)
: BaseVisitor (std::move (args.second)), _f (std::move (args.first))
{}
using BaseVisitor::visit;
virtual void visit (T &t) final override { _f (t); }
};
VisitorWrapper (Args &&args) : _args (std::move (args)) {}
// Add another visit() method to the visitor.
// _args will be moved over, so don't keep the old wrapper around.
template <typename T, typename F>
VisitorWrapper<Visitor<T, F>, std::pair<F, Args>> on (F &&f)
{
return VisitorWrapper<Visitor<T, F>, std::pair<F, Args>> (
std::make_pair (std::move (f), std::move (_args)));
}
// Returns the finished visitor.
// NOTE: The reference implementation has a bug that exposes this method even
// when BaseVisitor is still an abstract class. The C++11 standard states that
// "An abstract class shall not be used [...] as a function return type". GCC
// rejects the buggy code as expected, but Clang accepts the code as long as
// the method is not actually called. Maybe this is a bug in Clang?
template <typename T = BaseVisitor>
typename std::enable_if<std::is_constructible<T, Args>::value, T>::type
end_visitor ()
{
return T (std::move (_args));
}
};
// The entry point.
template <typename TargetVisitor>
VisitorWrapper<EmptyVisitor<TargetVisitor>, std::nullptr_t>
begin_visitor ()
{
return VisitorWrapper<EmptyVisitor<TargetVisitor>, std::nullptr_t> (nullptr);
}
} // namespace Rust
#endif
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