Question

In a given file if I have,

struct A { static int a; };
struct B { static int b; };
int A::a;
int B::b;

Then, I can always expect that A::a gets initialized before B::b. Now for the same file, take the template case,

template<typename T>
struct X { static T t; };
template<typename T>
T X<T>::t;

Suppose, X is instantiated with A and B and its static member is used arbitrarily somewhere in the code as, X<A>::t and X<B>::t, then what should be the order for initialization of template static member X<T>::t; ? Is it well defined ?

Answer 1

As long as the templates only have one definition (e.g. you only have one translation unit), it’s well-defined. The static members are initialized in the order that the template specializations are instantiated in contexts that require the definition of the static data member. From §14.7.1/1 [temp.inst] of the C++03 standard (emphasis mine):

Unless a class template specialization has been explicitly instantiated (14.7.2) or explicitly specialized (14.7.3), the class template specialization is implicitly instantiated when the specialization is referenced in a context that requires a completely-defined object type or when the completeness of the class type affects the semantics of the program. The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions or default arguments, of the class member functions, member classes, static data members and member templates; and it causes the implicit instantiation of the definitions of member anonymous unions. Unless a member of a class template or a member template has been explicitly instantiated or explicitly specialized, the specialization of the member is implicitly instantiated when the specialization is referenced in a context that requires the member definition to exist; in particular, the initialization (and any associated side-effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist.

§14.7.1/7 also states:

The implicit instantiation of a class template does not cause any static data members of that class to be implicitly instantiated.

However, things get more complicated when you have multiple translation units that define the template. §3.2/5 [basic.def.odr] states:

There can be more than one definition of a class type (clause 9), enumeration type (7.2), inline function with external linkage (7.1.2), class template (clause 14), non-static function template (14.5.5), static data member of a class template (14.5.1.3), member function of a class template (14.5.1.1), or template specialization for which some template parameters are not specified (14.7, 14.5.4) in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. Given such an entity named D defined in more than one translation unit, then

(list of conditions…)

If the definitions of D satisfy all these requirements, then the program shall behave as if there were a single definition of D. If the definitions of D do not satisfy these requirements, then the behavior is undefined.

Note that the standard doesn’t specify which definition is taken as the single definition, only that some definition is chosen. So, if multiple translation units instantiate the templates in different orders, there’s no guarantee what the order of initialization will be.