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In C++ given one std::variant type, can one add additional types to make another variant type?


By : Tolga Öztürk
Date : September 17 2020, 01:00 AM
wish helps you , You can create a meta-function to transform the variant type.
code :
template <class V, class T> struct variant_append_helper;

template <class... A, class T>
struct variant_append_helper<std::variant<A...>, T> {
    using type = std::variant<A..., T>;
};

template <class V, class T>
using variant_append = typename variant_append_helper<V, T>::type;

static_assert(std::is_same<std::variant<int, double, std::string>,
                           variant_append<std::variant<int, double>, std::string>>{});


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Construct a boost variant containing a value of the nth-type in the variant type index?


By : Vihar Bhagat
Date : March 29 2020, 07:55 AM
This might help you You need to use the variant::types typedef. This gives you an MPL compatible sequence which we can then use with mpl::at and a template to do our bidding. This does the trick:
code :
#include <string>
#include <boost/variant.hpp>
#include <boost/mpl/at.hpp>
#include <boost/mpl/int.hpp>

template<typename U, typename V>
void construct_in(V& v) {
  v = U();
  // modern
  // v = U{};
}

int main()
{
  typedef boost::variant<int, std::string> variant;
  typedef boost::mpl::at<variant::types, boost::mpl::int_<1>>::type pos;
  variant v;
  // use type deduction
  construct_in<pos>(v);
  // does not throw, does work
  std::string& s =boost::get<std::string>(v);
  return 0;
}
#include <string>
#include <vector>
#include <functional>

#include <boost/variant.hpp>
#include <boost/mpl/at.hpp>
#include <boost/mpl/int.hpp>
#include <boost/mpl/for_each.hpp>

typedef boost::variant<int, std::string> variant;
typedef variant::types types;
typedef std::vector< std::function<void(variant&)> > fvec;

template<typename U, typename V>
void construct_in(V& v) {
  v = U{};
}

struct build_and_add {
  fvec* funcs;
  template<typename T>
  void operator()(T) {
    funcs->push_back(&construct_in<T, variant>);
  }
};


int main()
{

  variant v;
  std::vector< std::function<void(variant&)> > funcs;

  // cannot use a lambda, would need to be polymorphic
  build_and_add f = {&funcs};
  boost::mpl::for_each<types>(f);

  // this is runtime!
  int i = 1;

  funcs[i](v);
  // does not throw, does work
  std::string& s =boost::get<std::string>(v);
  return 0;
}

In a template method, Is there a way to check whether the current type is int-variant or float-variant?


By : Angie
Date : March 29 2020, 07:55 AM
To fix the issue you can do To detect types:
http://en.cppreference.com/w/cpp/types/is_integral

How to copy an element of std::variant to a variable of another variant-type


By : user2793597
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , Apparently the requirement is that if the type isn't present in the target variant, throw an exception. We can do that by introducing a new type which is only exactly convertible to a specific target:
code :
template <typename T>
struct Exactly {
    template <typename U, std::enable_if_t<std::is_same_v<T, U>, int> = 0>
    operator U() const;
};
template <typename To, typename From>
To unsafe_variant_cast(From && from)
{
    return std::visit([](auto&& elem) -> To {
        using U = std::decay_t<decltype(elem)>;
        if constexpr (std::is_constructible_v<To, Exactly<U>>) {
            return To(std::forward<decltype(elem)>(elem));
        } else {
            throw std::runtime_error("Bad type");
        }
    }, std::forward<From>(from));
}
template <typename To, typename From>
To unsafe_variant_cast(From && from)
{
    return std::visit([](auto&& elem) -> To {
        using U = std::decay_t<decltype(elem)>;
        if constexpr (mp_contains<To, U>::value) {
            return To(std::forward<decltype(elem)>(elem));
        } else {
            throw std::runtime_error("Bad type");
        }
    }, std::forward<From>(from));
}

Could not convert variant if type (Array Variant) into type (OleStr)


By : user3218110
Date : March 29 2020, 07:55 AM
this will help I think you should set the Text of the barcode view itself instead of the text of the barcode of the barcode view, so:
code :
TfrxBarCodeView(barcode).Text := value;
TfrxBarCodeView(barcode).BarCode.Text := value;

C++ std::variant - Type traits to verify that the contained variant types meet some assumptions


By : INO
Date : March 29 2020, 07:55 AM
hope this fix your issue Let's say I have some set of types that all have a common parent: , You can use partial specialization:
code :
template<class>
struct checker : std::false_type {};

template<class... Ts> 
struct checker<std::variant<Ts...>> : 
    std::bool_constant<(has_some_tag<Ts>::value && ...)> {};
template<typename V>
struct some_other_type {
    static_assert(checker<V>::value);
};
template<class... Ts> 
struct checker<std::variant<Ts...>> : std::conjunction<has_some_tag<Ts>...> {};
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