How can I parametrize the execution policy of a standard library algorithm?

Question

I have this call to the sort C++ algorithm:

std::sort(std::execution::par_unseq, vec.begin(), vec.end());

Now I would like to parametrize the execution policy:

std::sort(executionPolicy, vec.begin(), vec.end());

However, std::execution::seq, std::execution::par, std::execution::par_unseq, std::execution::unseq are objects of different type, respectively: std::execution::sequenced_policy, std::execution::parallel_policy, std::execution::parallel_unsequenced_policy, std::execution::unsequenced_policy.

How can I use a variable to store objects belonging to different classes which do not share a base class?

Answer 1

The approach I've used for this has been to use std::variant and std::visit for this purpose.

using parallel_policy_holder = std::variant
                               <
                                   std::execution::sequenced_policy,
                                   std::execution::parallel_policy,
                                   std::execution::parallel_unsequenced_policy,
                                   std::execution::unsequenced_policy
                               >;

void foo(parallel_policy_holder policy, std::vector<int> const& input)
{
    std::visit
    (
        [&](auto policy_real) { std::sort(policy_real, input.begin(), input.end()); },
        policy
    );
}