这个sizeof表达式如何评估？ 为什么这样称呼？

Question

I came across with this code in an std::optional implementation :

template <class T, class U>
struct is_assignable
{
  template <class X, class Y>
  constexpr static bool has_assign(...) { return false; }

  template <class X, class Y, size_t S = sizeof((std::declval<X>() = std::declval<Y>(), true)) >
  // the comma operator is necessary for the cases where operator= returns void
  constexpr static bool has_assign(bool) { return true; }

  constexpr static bool value = has_assign<T, U>(true);
};

The part that I cant understand how it works or how it is evaluated is size_t S = sizeof((std::declval<X>() = std::declval<Y>(), true)) I know that if the assign operation fails it will fall back to the first definition of has_assign that returns false, but i don't know why it has the , true) part.

I did some test with structs that returns void on the assign operator and removing the , true part in sizeof gives me the same results.

Answer 1

In principle, the type of the expression std::declval<X>() = std::declval<Y>() (that is, the return type of the operator = involved) can be arbitrary, including an incomplete type or void . In such case, SFINAE wouldn't kick in, since the expression is valid. However, you'd then get an error from applying sizeof to an incomplete type. (Note that some compilers define sizeof(void) == 1 as an extension, but that cannot be universally relied upon).

Adding , true after the SFINAE expression fixes this by discarding the type of the assignment (whatever it is), and applying sizeof to true instead (which is perfectly valid).

As indicated by Barry in the comments, a more direct approach would be to use the type of the assignment in decltype instead of in sizeof , like this:

template <class X, class Y, class S = decltype(std::declval<X>() = std::declval<Y>()) >
constexpr static bool has_assign(bool) { return true; }

Answer 2

In order to apply sizeof() , you need a complete type. But returning a complete type isn't a requirement of assignability, hence:

sizeof((std::declval<X>() = std::declval<Y>(), true))
       ~~~~~~~~~~~~~~~~~~ expr ~~~~~~~~~~~~~~~~~~~~~

if the assignment is valid for those two types, then we have sizeof(expr) where the type of expr is bool (because true ). So if the assignment is valid, we get some real size . Otherwise, substitution failure.

---

But this is an unnecessarily cryptic way of writing this code. Moreover, it's not even correct because I could write a type like:

struct Evil {
    template <class T> Evil operator=(T&& ); // assignable from anything
    void operator,(bool);                    // mwahahaha
};

and now your sizeof() still doesn't work.

Instead, prefer simply:

class = decltype(std::declval<X>() = std::declval<Y>())

This accomplishes the same result - either substitution failure or not - without needed to care at all about what the type of the result is or to handle special cases.