c++ initialisation order of static members

Question

Just wondering if in the following the static members are initialised before the Foo class object is initialised. Since both are static variables, one a static member and the other a static global variable, initialisation order is not guaranteed or specified.

struct Foo
{
   Foo() { assert(a == 7); }

   
   static inline int a = 7;
};

Foo foo;

int main()
{

}

So the initialisation order between the global Foo and the static class member is not defined, you would think there is no guarantee. However, I'm thinking that before a Foo is instantiated that the Foo class would need to be completed/initialised first, and so in that case that there might be a guarantee that the static member variable would be initialised first.

Answer 1

I'm thinking that before a Foo is instantiated that the Foo class would need to be completed/initialised first

That is not generally the case, so you should be careful with that assumption.

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However, in your specific example, the order of initialization is guaranteed. The initializer of a is a constant expression and therefore a will be constant-initialized. Constant-initialization is guaranteed to happen before any dynamic initialization, which the initialization of foo is.

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Even if a was not constant-initialized, there wouldn't be an issue here, because foo is defined after a in the same translation unit, foo is not an inline or template variable and because Foo is not a template. If either of these requirements were not fulfilled, there could be problems in the ordering guarantees.