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On the [cppreference page for `std::async`](https://en.cppreference.com/w/cpp/thread/async), there's an example where the object for a PMF invocation is passed by value. So clearly, that is okay; in fact there is nothing on the cppreference page saying it has to be a reference.

However, in your example, you've only got passing by value (`x`) and passing by pointer (`&x`); you can also pass by reference using `std::ref(x)`. Here, I modified your example to show this to better effect:

```cpp
#include <future>
#include <iostream>
#include <thread>

struct X {
    X() noexcept {
        std::cerr << "X::X()\n";
    }

    X(X const&) noexcept {
        std::cerr << "X::X(X const&)\n";
    }

    ~X() noexcept {
        std::cerr << "X::~X()\n";
    }

    void add(int num1, int num2) noexcept {
        int result = num1 + num2;

        std::cout << "The result is " << result << "\n";
    }
};

int main() {
    X x;
    std::cerr << "\nBy reference:\n";
    std::async(&X::add, std::ref(x), 42, 5).get();
    std::cerr << "\nBy pointer:\n";
    std::async(&X::add, &x, 42, 5).get();
    std::cerr << "\nBy value:\n";
    std::async(&X::add, x, 42, 5).get();
    std::cerr << "\nAll done!\n";
}
```