Convert std::variant to another std::variant with super-set of types

问题

I have a std::variant that I'd like to convert to another std::variant that has a super-set of its types. Is there a way of doing it than that allows me to simply assign one to the other?

template <typename ToVariant, typename FromVariant>
ToVariant ConvertVariant(const FromVariant& from) {
    ToVariant to = std::visit([](auto&& arg) -> ToVariant {return arg ; }, from);
    return to;
}

int main()
{
    std::variant<int , double> a;
    a = 5;
    std::variant <std::string, double, int> b;
    b = ConvertVariant<decltype(b),decltype(a)>(a);
    return 0;
}

I'd like to be able to simply write b = a in order to do the conversion rather than going through this complex casting setup. Without polluting the std namespace.

Edit: Simply writing b = a gives the following error:

error C2679: binary '=': no operator found which takes a right-hand operand of type 'std::variant<int,double>' (or there is no acceptable conversion) 

note: while trying to match the argument list '(std::variant<std::string,int,double>, std::variant<int,double>)'

回答1:

This is an implementation of Yakk's second option:

template <class... Args>
struct variant_cast_proxy
{
    std::variant<Args...> v;

    template <class... ToArgs>
    operator std::variant<ToArgs...>() const
    {
        return std::visit([](auto&& arg) -> std::variant<ToArgs...> { return arg ; },
                          v);
    }
};

template <class... Args>
auto variant_cast(const std::variant<Args...>& v) -> variant_cast_proxy<Args...>
{
    return {v};
}

You might want to fine tune it for forwarding semantics.

And as you can see its use is simple:

std::variant<int, char> v1 = 24;
std::variant<int, char, bool> v2;

v2 = variant_cast(v1);

回答2:

Options:

Write your own variant type, possibly inheriting from std::variant, that implements operator= and construction the way you want. Some work has to be done, because variant's constructors can do SFINAE tricks that might not work properly with your variant type; to that end, you want to do some SFINAE forwarding to base-variant yourself instead of naked using declarations.
Write a better ConvertVariant that doesn't require the source/destination types to be listed. You would return a convert helper template type that holds the source variant which has an operator std::variant<Ts...>()&& that calls something very much like your ConvertVariant.

来源：https://stackoverflow.com/questions/47203255/convert-stdvariant-to-another-stdvariant-with-super-set-of-types

标签

c++

type-conversion

c++17

variant