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Building a std::variant of pointers of types in the alternatives of another std::variant

September 19, 2024

I’m given a std::variant of several types. I would like to write a trait that gives another std::variant of the pointer of the types of the first.

using MyVariant = std::variant<int, float>;

using MyVariantOfPointers = pointer_of_v<MyVariant>;

such that MyVariantOfPointer is std::variant<int*, float*>.
I tried to play with std::variant_size_v and std::variant_alternative_t but I do not know how to build the new type recursively.

>Solution :

You can use partial specialization to write a trait to solve this.

// General case
template<class T>
struct VariantToPtrVariant;

// Partial specialization when used with std::variant
// Extracts the template arguments from the `std::variant` and 
//  produces a new `std::variant` with those types, with an added `*`
template<class ... T>
struct VariantToPtrVariant<std::variant<T...>> {
    using type = std::variant<T*...>;
};

// Helper variable template
template<class T>
using VariantToPtrVariant_t = typename VariantToPtrVariant<T>::type;

Usage :

using MyVariant = std::variant<int, float>;
using MyVariantOfPointers = VariantToPtrVariant_t<MyVariant>;

Complete example (https://godbolt.org/z/qaTxo4nzc) :

#include <type_traits>
#include <variant>

template<class T>
struct VariantToPtrVariant;

template<class ... T>
struct VariantToPtrVariant<std::variant<T...>> {
    using type = std::variant<T*...>;
};

template<class T>
using VariantToPtrVariant_t = typename VariantToPtrVariant<T>::type;

using MyVariant = std::variant<int, float>;
using MyVariantOfPointers = VariantToPtrVariant_t<MyVariant>;

static_assert(std::is_same_v<MyVariantOfPointers, std::variant<int*, float*>>);