94. May library implementors add template parameters to Standard Library classes?

Section: 20.5.5 [conforming] Status: NAD Submitter: Matt Austern Opened: 1998-01-22 Last modified: 2016-02-10

Priority: Not Prioritized

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Discussion:

Is it a permitted extension for library implementors to add template parameters to standard library classes, provided that those extra parameters have defaults? For example, instead of defining template <class T, class Alloc = allocator<T> > class vector; defining it as template <class T, class Alloc = allocator<T>, int N = 1> class vector;

The standard may well already allow this (I can't think of any way that this extension could break a conforming program, considering that users are not permitted to forward-declare standard library components), but it ought to be explicitly permitted or forbidden.

comment from Steve Cleary via comp.std.c++:

I disagree [with the proposed resolution] for the following reason: consider user library code with template template parameters. For example, a user library object may be templated on the type of underlying sequence storage to use (deque/list/vector), since these classes all take the same number and type of template parameters; this would allow the user to determine the performance tradeoffs of the user library object. A similar example is a user library object templated on the type of underlying set storage (set/multiset) or map storage (map/multimap), which would allow users to change (within reason) the semantic meanings of operations on that object.

I think that additional template parameters should be forbidden in the Standard classes. Library writers don't lose any expressive power, and can still offer extensions because additional template parameters may be provided by a non-Standard implementation class:

 
   template <class T, class Allocator = allocator<T>, int N = 1>
   class __vector
   { ... };
   template <class T, class Allocator = allocator<T> >
   class vector: public __vector<T, Allocator>
   { ... };

Proposed resolution:

Add a new subclause [presumably 17.4.4.9] following 20.5.5.12 [res.on.exception.handling]:

17.4.4.9 Template Parameters

A specialization of a template class described in the C++ Standard Library behaves the same as if the implementation declares no additional template parameters.

Footnote: Additional template parameters with default values are thus permitted.

Add "template parameters" to the list of subclauses at the end of 20.5.5 [conforming] paragraph 1.

[Kona: The LWG agreed the standard needs clarification. After discussion with John Spicer, it seems added template parameters can be detected by a program using template-template parameters. A straw vote - "should implementors be allowed to add template parameters?" found no consensus ; 5 - yes, 7 - no.]

Rationale:

There is no ambiguity; the standard is clear as written. Library implementors are not permitted to add template parameters to standard library classes. This does not fall under the "as if" rule, so it would be permitted only if the standard gave explicit license for implementors to do this. This would require a change in the standard.

The LWG decided against making this change, because it would break user code involving template template parameters or specializations of standard library class templates.