struct A { static int a; };
struct B { static int b; };
int A::a;
int B::b;
然后,我总是可以期望A :: a在B :: b之前被初始化.现在对于同一个文件,采取模板案例,
template<typename T>
struct X { static T t; };
template<typename T>
T X<T>::t;
假设,X用A和B实例化,并且其静态成员在代码中的某处任意使用,如X< A> :: t和X< B> :: t,则应该是模板静态成员初始化的顺序X< T> ::吨; ？它定义明确吗？
解决方法
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还规定：
The implicit instantiation of a class template does not cause any static data members of that class to be implicitly instantiated.
但是,当您有多个定义模板的翻译单元时,事情变得更加复杂. §3.2/ 5 [basic.def.odr]指出：
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.
请注意,标准没有指定将哪个定义作为单个定义,只是选择了某个定义.因此,如果多个翻译单元以不同的顺序实例化模板,则无法保证初始化的顺序.
