libcruft-util/typeidx.hpp

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/*
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* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/.
*
* Copyright 2017-2019 Danny Robson <danny@nerdcruft.net>
*/
#pragma once
#include <atomic>
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#include <variant>
namespace cruft {
namespace detail {
template <typename TagT>
int typeidx_next (void)
{
static std::atomic<int> counter;
return ++counter;
}
}
/// Return a globally unique runtime ID for a given type (namespaced by a
/// tag type).
///
/// This is intended to be used as a lightweight type check for variants
/// and such without requiring RTTI.
///
/// The identifier is constructed at runtime and is not guaranteed to be
/// stable across executions (particularly in the case of threads and
/// other non-determinism). However it _is_ threadsafe to call this.
///
/// The value 0 will never be returned from this function and can be used
/// to indicate an 'unknown' state.
///
/// The range of identifiers is _probably_ contiguous. This should not
/// be relied upon for correctness, but may be used for performance
/// concerns.
///
/// \tparam T The type to register-and-retrieve the ID for
/// \tparam TagT A namespacing type; different tags will have different
/// sets of IDs. The default Tag is void.
/// \return The unique ID of the type
template <typename T, typename TagT = void>
int
typeidx (void)
{
static auto id = detail::typeidx_next<TagT> ();
return id;
}
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/// Returns the typeidx of the contained value within a variant.
///
/// May throw std::bad_variant_access if the variant is
/// valueless_by_exception.
template <typename TagT=void, typename ...ComponentT>
int
value_typeidx (std::variant<ComponentT...> const &val)
{
return std::visit (
[] <typename ValueT> (ValueT const &) {
return typeidx<ValueT> ();
},
val
);
}
}