Defining a static member

Question

Consider the following code:

#include <iostream > using namespace std; class A { private: int x; public: A(int _x) { x = _x; } int get() { return x; } }; class B { static A a; public: static int get() { return a.get(); } }; A B::a(0); int main(void) { B b; cout << b.get(); return 0; }

My book says:

If we do not use the line of code A B::a(0),there is a compiler error because static member a is not defined in B. To fix the error, we need to explicitly define a.

However, I thought of initializing object a as static A a(0); but it gives me a compiler error. Can someone explain why I can't initialize object a in the manner I described, and why it is necessary to initialize it as they had given it in book.

Are you placing static A a(0); inside or outside the class definition? Why it could be wrong changes depending on the location. — user4581301
– user4581301, Commented Jun 29, 2021 at 2:48
Side note: what you can get away with in static members is something of a moving target. Every new revision of the C++ Standard relaxes some of the restrictions and adds a bit more functionality. — user4581301
– user4581301, Commented Jun 29, 2021 at 2:50
@user4581301 I am trying to place static A a(0) inside the class definition. Can you please tell why it would be wrong to place it here? — loveofprogramming
– loveofprogramming, Commented Jun 29, 2021 at 2:57

Ted Lyngmo · Accepted Answer · 2021-06-29 02:51:06Z

1

If you want to define a inline, you need to inline it, which is possible from C++17:

class B { inline static A a{0}; // or inline static A a = 0; public: static int get() { return a.get(); } };

Demo

answered Jun 29, 2021 at 2:51

Ted Lyngmo

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6 Comments

loveofprogramming Over a year ago

Buy why do I need to use the keyword inline? Also can I define it as inline static A(0) instead of curly brackets? Also, do other standards of C++ allow this?

Ted Lyngmo Over a year ago

@loveofprogramming "But why": Basically just because the language requires it. I don't know the reason why. That's just the way it is. No, not without curly brackets. Either curly brackets or =. No, the standards before C++17 doesn't allow it.

Ted Lyngmo Over a year ago

@loveofprogramming eerorika's answer to a similar question deals with the "why" to some extent.

Ted Lyngmo Over a year ago

@user4581301 The compiler could probably pick the first definition it finds when linking and the standard could just say it's UB to have conflicting definitions. I guess inline is required just to make a distinction between the two ways of defining static members but I don't know if there would be any drawbacks to just drop the requirement for inline in the future now that both member functions and variables can be inline. A static variable definition could probably be implicitly inline just like functions that are defined in class definitions are implicitly inline.

user4581301 Over a year ago

Agreed, it could, but if that were easy, the static initialization order fiasco wouldn't be such a fiasco.

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