Why Does My Static Member in a Template Class Not Initialize When Used?

Static Member Initialization in Template Classes

When initializing static members of a template class, it's essential to understand how template instantiation affects member definition.

In the provided code, the static member initialization is wrapped inside a nested helper struct to avoid issues with non-templated classes.

However, when the enclosing class is parameterized by a template, the nested initialization struct may not be instantiated unless the helper object is accessed in the main code.

Understanding Implicit Template Instantiation

The behavior of static member initialization in templates is governed by the ISO/IEC C 2003 standard (14.7.1). According to this standard:

• Unless explicitly instantiated or specialized, the member of a class template is implicitly instantiated when referenced in a context requiring its definition.
• The initialization of a static data member occurs only if the data member itself is used in a way that requires its existence.

Code Analysis

• [1] commented and [2] commented: Since there are no references to the static data members, their definitions (including declarations and instantiations) are not created, resulting in no side effects.
• [1] uncommented: B::getB() references B::mB, requiring its existence. However, B::mInit is not used, leading to its non-instantiation. The constructor for B::InitHelper is not executed, resulting in B::mB not being assigned.
• [1] and [2] uncommented: This worked in your case due to unspecified behavior regarding the order of initialization.
• [1] commented and [2] uncommented: B::mB is referenced through B::getHelper(). However, since B::mInit is not constructed first, its constructor attempts to assign to a non-yet-constructed string object, resulting in a segfault.

Conclusion and Elegant Solution

The standard behavior implies that static data members are not automatically initialized unless they are used. To avoid this issue, consider using explicit specialization to explicitly define the static data members before referencing them. This enforces ordered initialization, as explicit specializations are treated as regular declarations.

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