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// Variant wrapper class.
#pragma once
//local headers
//third party headers
#include <boost/blank.hpp>
#include <boost/mpl/begin_end.hpp>
#include <boost/mpl/distance.hpp>
#include <boost/mpl/find.hpp>
#include <boost/mpl/vector.hpp>
#include <boost/none_t.hpp>
#include <boost/variant/apply_visitor.hpp>
#include <boost/variant/get.hpp>
#include <boost/variant/static_visitor.hpp>
#include <boost/variant/variant.hpp>
//standard headers
#include <stdexcept>
#include <type_traits>
#include <utility>
//forward declarations
namespace tools
{
[[noreturn]] inline void variant_static_visitor_blank_err()
{ throw std::runtime_error("variant: tried to visit an empty variant."); }
[[noreturn]] inline void variant_unwrap_err()
{ throw std::runtime_error("variant: tried to access value of incorrect type."); }
////
// variant: convenience wrapper around boost::variant with a cleaner interface
// - the variant is 'optional' - an empty variant will evaluate to 'false' and an initialized variant will be 'true'
///
template <typename ResultT>
struct variant_static_visitor : public boost::static_visitor<ResultT>
{
/// provide visitation for empty variants
/// - add this to your visitor with: using variant_static_visitor::operator();
[[noreturn]] ResultT operator()(const boost::blank) { variant_static_visitor_blank_err(); }
[[noreturn]] ResultT operator()(const boost::blank) const { variant_static_visitor_blank_err(); }
};
template <typename... Types>
class variant final
{
using VType = boost::variant<boost::blank, Types...>;
public:
//constructors
/// default constructor
variant() = default;
variant(boost::none_t) : variant{} {} //act like boost::optional
/// construct from variant type (use enable_if to avoid issues with copy/move constructor)
template <typename T,
typename std::enable_if<
!std::is_same<
std::remove_cv_t<std::remove_reference_t<T>>,
variant<Types...>
>::value,
bool
>::type = true>
variant(T &&value) : m_value{std::forward<T>(value)} {}
//overloaded operators
/// boolean operator: true if the variant isn't empty/uninitialized
explicit operator bool() const noexcept { return !this->is_empty(); }
//member functions
/// check if empty/uninitialized
bool is_empty() const noexcept { return m_value.which() == 0; }
/// check the variant type
template <typename T>
bool is_type() const noexcept { return this->index() == this->type_index_of<T>(); }
/// try to get a handle to the embedded value (return nullptr on failure)
template <typename T>
T* try_unwrap() noexcept { return boost::strict_get< T>(&m_value); }
template <typename T>
const T* try_unwrap() const noexcept { return boost::strict_get<const T>(&m_value); }
/// get a handle to the embedded value
template <typename T>
T& unwrap()
{
T *value_ptr{this->try_unwrap<T>()};
if (!value_ptr) variant_unwrap_err();
return *value_ptr;
}
template <typename T>
const T& unwrap() const
{
const T *value_ptr{this->try_unwrap<T>()};
if (!value_ptr) variant_unwrap_err();
return *value_ptr;
}
/// get the type index of the currently stored type
int index() const noexcept { return m_value.which(); }
/// get the type index of a requested type (compile error for invalid types) (boost::mp11 is boost 1.66.0)
template <typename T>
static constexpr int type_index_of() noexcept
{
using types = boost::mpl::vector<boost::blank, Types...>;
using elem = typename boost::mpl::find<types, T>::type;
using begin = typename boost::mpl::begin<types>::type;
return boost::mpl::distance<begin, elem>::value;
}
/// check if two variants have the same type
static bool same_type(const variant<Types...> &v1, const variant<Types...> &v2) noexcept
{ return v1.index() == v2.index(); }
/// apply a visitor to the variant
template <typename VisitorT>
typename VisitorT::result_type visit(VisitorT &&visitor)
{
return boost::apply_visitor(std::forward<VisitorT>(visitor), m_value);
}
template <typename VisitorT>
typename VisitorT::result_type visit(VisitorT &&visitor) const
{
return boost::apply_visitor(std::forward<VisitorT>(visitor), m_value);
}
private:
//member variables
/// variant of all value types
VType m_value;
};
} //namespace tools
