什么是可能评估的表达式？

Question

I am reading about ODR-use and I encountered this:

a variable x in a potentially-evaluated expression ex is odr-used unless both of the following are true:

What is a potentially-evaluated expression ?

Update: I might just found the answer when rolling down the page:

In the definitions above, potentially-evaluated means the expression is not an unevaluated operand (or its subexpression), such as the operand of sizeof and a set of potential results of an expression e is a (possibly empty) set of id-expressions that appear within e, combined as follows:

But not sure if this is what it means in general + I do not really understand their explanation anyway. Also, why to say potential result instead of just result ?

Answer 1

In simplest terms, potentially-evaluated is exactly what it says it is: the expression has a chance to be evaluated. This includes anything except sizeof(ex) , decltype(ex) , typeid(ex) and noexcept(ex) . Other than in these contexts, ex is potentially-evaluated.

In relation to odr-used, it means x is considered odr-used only if ex is potentially-evaluated. That is to say, the necessary condition for x to be odr-used is that ex must be potentially-evaluated. This enables things such as

struct S
{
    static float f;  // declared but not defined
};

decltype(&S::f) p1;  // since &S::f isn't potentially evaluated, this is well-formed

float* p2 = &S::f;  // this is ill-formed