检查变体在编译时是否具有价值

Question

Can't use variant.index() in constexpr statements so need to iterate variant and return true if it can cast to some type or false if it's emtpy or contains zero value. Try this code, but seems index sequence is not variadic type and... operator not available in this case.

template <typename T>
bool has_value(T value) noexcept {
    if constexpr (std::is_convertible_v <T, bool>) {
        return value;
    }
    else if constexpr (is_variant_v<T>) {
        constexpr size_t N = std::variant_size_v<decltype(value)>;
        using variant_sequence = typename std::make_index_sequence<0, N-1>::type;
        bool __has_value = (( std::get<variant_sequence>(value), true) || variant_sequence... ) ;
        return __has_value;
    }
    return false;
}

Answer 1

I think you want:

template <typename T>
bool has_value(T value) noexcept {
    if constexpr (std::is_convertible_v <T, bool>) {
        return value;
    } else if constexpr (is_variant_v<T>) {
        return std::visit([](const auto& elem){ return has_value(elem); }, value);
    }
    return false;
}