Cleaner way to specify type to get from a std::variant?

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I've got code that can be simplified to

std::variant<float, int> v[2] = foo();
int a = std::get<decltype(a)>(v[0]);
float b = std::get<decltype(b)>(v[1]);

Obviously this can go throw if foo() returns the wrong variants, but that's not my problem here. (The real code has a catch). My problem is that the decltype(a) violates the Don't Repeat Yourself principle.

Is there a cleaner way to initialize a and b, and still throw if the types do not match expectations? In particular, I don't want a static_cast<int>(std::get<float>(v)) if the variant contains a float while I'm trying to initialize an int.

std-variant c++17 c++
3 Answers

You could wrap your call to get in a template that implicitly converts to the target type.

template<typename... Ts>
struct variant_unwrapper {
    std::variant<Ts...> & var;
    template <typename T>
    operator T() { return std::get<T>(var); }
};

See it on coliru

user_26108100

IMO it would be nice to allow template deduction to take over, so providing a helper function should do the job:

template<typename T, typename...VariantParams>
void get_from(const std::variant<VariantParams...>& v, T& value)
{
    value = ::std::get<T>(v);
}

int a;

get_from(v[0], a);
user_13874380

As @paulo says in the comments, seems like the DRY solution is to use auto for the declaration, changing:

int a = std::get<decltype(a)>(v[0]);

to:

auto a = std::get<int>(v[0]);

You only name the type (int) and the variable (a) once each. Doesn't work if you separate declaration and initialization, so you'd still need:

int a;
...
a = std::get<decltype(a)>(v[0]);

in that case, but if you write all your C++ code deferring declarations until the point of definition, it's not needed often.

user_3646960
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