Question

How to access non-static class member?

class A
{
    int value_ = 0;//I want to access this member in class inheriting from A
};
template<class X = A>
class Y :X
{
static_assert(value_ == 0,"Non-zero not allowed");//here I want to access value_ from X which is A by default. Is this possible?
};

Answer 1

What you are specifically doing is not possible, for several reasons.

First, static_assert requires it’s argument to be a compile-time constant expression. value_ is most assuredly not.

You could try to make it a constant expression by labeling it constexpr. But C++11 doesn’t allow constexpr for non-static data members. And even if it did, that’s not going to help because value_ is a non-static member. As such, it doesn’t exist yet. It only exists when there is an actual class instance, something that has a this pointer. Your static_assert expression doesn’t create one of those objects, so there’s no way to access it.

Therefore, in order to make this “work” (to the extent that “working” does something meaningful), you must:

1. Give A a constexpr constructor, so that you can create a constexpr instantiation of it.
2. Your static_assert must actually create a type using the constexpr constructor and access the data member in question.

Now, if you do this, you’re going to find that it doesn’t give you what you want (based on this statement, since your question didn’t explain what exactly it is you’re trying to accomplish):

Secondly, by every time I meant every time object of this class is created, and as a X is a template param it is a constant expression.

Note that static_assert is static. Whereas “every time object of this class is created” is a runtime event. You cannot perform a static test on something that happens at runtime.

What you want is regular assert, not static_assert.