Using a constant variable as the size of an array

Question

How come the following code snippet is compiled with no error:

void func(){ const int s_max{ 10 }; int m_array[s_max]{0}; } int main() { const int s_max{ 10 }; int m_array[s_max]{0}; return 0; }

but when I try to define the same array within a class scope, I get the following error:

class MyClass { const int s_max{ 10 }; int m_array[s_max]{0}; // error: invalid use of non-static data member 's_max' };

Why does s_max need to be static within the class?

I could not find a convincing answer to my question in other similar posts.

The length of an array must be a constant expression and const is necessary but not sufficient for a constant expression. — Henri Menke
– Henri Menke, Commented Aug 23, 2019 at 2:13
I have some bad news: even though the first snippet "compiled with no error", it is not valid C++. Your compiler is just doing you a favor, and allowing something that's not standard C++. — Sam Varshavchik
– Sam Varshavchik, Commented Aug 23, 2019 at 2:14
@SamVarshavchik I think the 1st case should be fine; s_max is a constant expression. has integral or enumeration type and refers to a complete non-volatile const object, which is initialized with a constant expression — songyuanyao
– songyuanyao, Commented Aug 23, 2019 at 2:20

songyuanyao · Accepted Answer · 2019-08-23 02:12:25Z

As a non-static data member, it might be initialized with different values via different initialization ways (constructors (member initializer lists), default member initializer, aggregate initialization, etc). Then its value won't be determined until the initialization. But the size of raw array must be fixed and known at compile-time. e.g.

class MyClass { const int s_max{ 10 }; int m_array[s_max]{0}; // error: invalid use of non-static data member 's_max' MyClass(...some arguments...) : s_max {20} {} MyClass(...some other arguments...) : s_max {30} {} };

I see, so when we make it static (and we have to also assign it a value), it will be known at compile time and won't depend on different initializations, right?
@Monaj Yes, static members can't be initialized in different uncertain ways.

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