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422. explicit specializations of member functions of class templates

Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It has been suggested that 17.4.3.1, p1 may or may not allow programs to explicitly specialize members of standard templates on user-defined types. The answer to the question might have an impact where library requirements are given using the "as if" rule. I.e., if programs are allowed to specialize member functions they will be able to detect an implementation's strict conformance to Effects clauses that describe the behavior of the function in terms of the other member function (the one explicitly specialized by the program) by relying on the "as if" rule.

Proposed resolution:

Add the following sentence to 16.4.5.3 [reserved.names], p1:

It is undefined for a C++ program to add declarations or definitions to namespace std or namespaces within namespace std unless otherwise specified. A program may add template specializations for any standard library template to namespace std. Such a specialization (complete or partial) of a standard library template results in undefined behavior unless the declaration depends on a user-defined type of external linkage and unless the specialization meets the standard library requirements for the original template.¹⁶⁸⁾ A program has undefined behavior if it declares

• an explicit specialization of any member function of a standard library class template, or
• an explicit specialization of any member function template of a standard library class or class template, or
• an explicit or partial specialization of any member class template of a standard library class or class template.

A program may explicitly instantiate any templates in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.

[Kona: straw poll was 6-1 that user programs should not be allowed to specialize individual member functions of standard library class templates, and that doing so invokes undefined behavior. Post-Kona: Martin provided wording.]

[Sydney: The LWG agrees that the standard shouldn't permit users to specialize individual member functions unless they specialize the whole class, but we're not sure these words say what we want them to; they could be read as prohibiting the specialization of any standard library class templates. We need to consult with CWG to make sure we use the right wording.]