emplace pointer as proper type into std::variant in template-CodePudding

Is it possible to emplace some void* pointer into variant with a runtime generated index.

Can this nice solution (from here link), be updated to get index and pointer to emplace?

#include <variant>

template <typename... Ts, std::size_t... Is>
void next(std::variant<Ts...>& v, std::index_sequence<Is...>)
{
    using Func = void (*)(std::variant<Ts...>&);
    Func funcs[] = {
         [](std::variant<Ts...>& v){ v.template emplace<(Is   1) % sizeof...(Is)>(); }...
    };
    funcs[v.index()](v);
}

template <typename... Ts>
void next(std::variant<Ts...>& v)
{
    next(v, std::make_index_sequence<sizeof...(Ts)>());
}

It is possible to get the required data type, but don't think it helps anyway

           Func funcs[] = {
                [](MessageTypesVariant& v) {
                    auto y = std::get<(Is)>(v);
                    auto z = std::forward<decltype(y)>(y);
                    v.template emplace<(Is)>(); }...
            };

CodePudding user response：

You can do it like this, unpack Ts... and check if the type matches.

template <typename T, typename... Ts>
bool set(std::variant<Ts*...>& v, int typeIndex, void* ptrToData) {
    constexpr static auto idx = index_v<T, Ts...>;
    if (idx == typeIndex) {
        v.template emplace<idx>(static_cast<T*>(ptrToData));
        return true;
    }
    return false;
}

template <typename... Ts>
void set(std::variant<Ts*...>& v, int typeIndex, void* ptrToData) {
    (set<Ts, Ts...>(v, typeIndex, ptrToData) || ...);
}

Here, you need a way to index a type from a typelist, index_v, a common tech used in typelist.

// index
inline constexpr std::size_t npos = -1;

template <typename T, typename... Ts>
struct index : std::integral_constant<std::size_t, npos> {};

template <typename T, typename... Ts>
inline constexpr std::size_t index_v = index<T, Ts...>::value;

template <typename T, typename... Ts>
struct index<T, T, Ts...> : std::integral_constant<std::size_t, 0> {};

template <typename T, typename Head, typename... Tail>
class index<T, Head, Tail...> {
    static constexpr std::size_t tmp = index_v<T, Tail...>;

public:
    static constexpr std::size_t value = tmp == npos ? tmp : tmp   1;
};

Demo