variant_helpers.h

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#pragma once
 
#include <string>
#include <variant>
#include <vector>
 
#include <boost/mp11.hpp>
 
 
namespace knp::meta
{
 
template <typename SupportedTypes, typename AllVariants, typename ToContainer>
void load_from_container(const std::vector<AllVariants> &from_container, ToContainer &to_container)
{
    to_container.clear();
    to_container.reserve(from_container.size());
 
    for (const auto &p : from_container)
    {
        std::visit(
            [&to_container](auto &arg)
            {
                using T = std::decay_t<decltype(arg)>;
                if constexpr (boost::mp11::mp_find<SupportedTypes, T>{} != boost::mp11::mp_size<SupportedTypes>{})
                {
                    to_container.push_back(typename ToContainer::value_type{arg});
                }
            },
            p);
    }
}
 
 
template <class... FromArgs>
struct variant_cast_proxy
{
    std::variant<FromArgs...> v;
    template <class... ToArgs>
    operator std::variant<ToArgs...>() const
    {
        return std::visit([](auto &&arg) -> std::variant<ToArgs...> { return arg; }, v);
    }
};
 
 
template <class... Args>
static auto variant_cast(const std::variant<Args...> &v) -> variant_cast_proxy<Args...>
{
    return {v};
}
 
}  // namespace knp::meta