Юрий Борисов

#include <functional>
#include <string>

template <typename F>
struct HandlerTraits;

template <typename R, typename ...Args>
struct HandlerTraits<std::function<R(Args...)>>
{
    using ReturnType = R;
    using ArgTypes = std::tuple<Args...>;
    static constexpr std::size_t ArgCount = sizeof...(Args);
    template <std::size_t N>
    using NthArg = std::tuple_element_t<N, ArgTypes>;
};


template <typename T>
void processEvent(T const& handler)
{   
    using Traits = HandlerTraits<T>;

    static_assert(std::is_same_v<typename Traits::ReturnType, void>);
    static_assert(Traits::ArgCount == 2);
    static_assert(std::is_same_v<typename Traits::template NthArg<0>, std::string*>);
    static_assert(std::is_same_v<typename Traits::template NthArg<1>, int const&>);
}

int main()
{
    processEvent(std::function<void(std::string*, int const&)>{});
}