This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-11-11

226. Default template arguments for function templates

[Voted into WP at April 2003 meeting.]

The prohibition of default template arguments for function templates is a misbegotten remnant of the time where freestanding functions were treated as second class citizens and required all template arguments to be deduced from the function arguments rather than specified.

The restriction seriously cramps programming style by unnecessarily making freestanding functions different from member functions, thus making it harder to write STL-style code.

Suggested resolution:

Replace

A default template-argument shall not be specified in a function template declaration or a function template definition, nor in the template-parameter-list of the definition of a member of a class template.

by

A default template-argument shall not be specified in the template-parameter-list of the definition of a member of a class template.

The actual rules are as stated for arguments to class templates.

Notes from 10/00 meeting:

The core language working group was amenable to this change. Questions arose, however, over the interaction between default template arguments and template argument deduction: should it be allowed or forbidden to specify a default argument for a deduced parameter? If it is allowed, what is the meaning: should one or the other have priority, or is it an error if the default and deduced arguments are different?

Notes from the 10/01 meeting:

It was decided that default arguments should be allowed on friend declarations only when the declaration is a definition. It was also noted that it is not necessary to insist that if there is a default argument for a given parameter all following parameters have default arguments, because (unlike in the class case) arguments can be deduced if they are not specified.

Note that there is an interaction with issue 115.

Proposed resolution (revised October 2002):

1. In 13.2 [temp.param] paragraph 9, replace

   A default template-argument may be specified in a class template declaration or a class template definition. A default template-argument shall not be specified in a function template declaration or a function template definition, nor in the template-parameter-list of the definition of a member of a class template.

   with

   A default template-argument may be specified in a template declaration. A default template-argument shall not be specified in the template-parameter-lists of the definition of a member of a class template that appears outside of the member's class.

2. In 13.2 [temp.param] paragraph 9, replace

   A default template-argument shall not be specified in a friend template declaration.

   with

   A default template-argument shall not be specified in a friend class template declaration. If a friend function template declaration specifies a default template-argument, that declaration shall be a definition and shall be the only declaration of the function template in the translation unit.

3. In 13.2 [temp.param] paragraph 11, replace

   If a template-parameter has a default template-argument, all subsequent template-parameters shall have a default template-argument supplied.

   with

   If a template-parameter of a class template has a default template-argument, all subsequent template-parameters shall have a default template-argument supplied. [Note: This is not a requirement for function templates because template arguments might be deduced (13.10.3 [temp.deduct]).]

4. In 13.10 [temp.fct.spec] paragraph 1, replace

   Template arguments can either be explicitly specified when naming the function template specialization or be deduced (13.10.3 [temp.deduct]) from the context, e.g. from the function arguments in a call to the function template specialization.

   with

   Template arguments can be explicitly specified when naming the function template specialization, deduced from the context (13.10.3 [temp.deduct]), e.g., deduced from the function arguments in a call to the function template specialization), or obtained from default template arguments.

5. In 13.10.2 [temp.arg.explicit] paragraph 2, replace

   Trailing template arguments that can be deduced (13.10.3 [temp.deduct]) may be omitted from the list of explicit template-arguments.

   with

   Trailing template arguments that can be deduced (13.10.3 [temp.deduct]) or obtained from default template-arguments may be omitted from the list of explicit template-arguments.

6. In 13.10.3 [temp.deduct] paragraph 1, replace

   The values can be either explicitly specified or, in some cases, deduced from the use.

   with

   The values can be explicitly specified or, in some cases, be deduced from the use or obtained from default template-arguments.

7. In 13.10.3 [temp.deduct] paragraph 4, replace

   The resulting substituted and adjusted function type is used as the type of the function template for template argument deduction. When all template arguments have been deduced, all uses of template parameters in nondeduced contexts are replaced with the corresponding deduced argument values. If the substitution results in an invalid type, as described above, type deduction fails.

   with

   The resulting substituted and adjusted function type is used as the type of the function template for template argument deduction. If a template argument has not been deduced, its default template argument, if any, is used. [Example:

       template <class T, class U = double>
       void f(T t = 0, U u = 0);
   
       void g()
       {
           f(1, 'c');         // f<int,char>(1,'c')
           f(1)               // f<int,double>(1,0)
           f();               // error: T cannot be deduced
           f<int>();          // f<int,double>(0,0)
           f<int,char>();     // f<int,char>(0,0)
       }
   

   ---end example]

   When all template arguments have been deduced or obtained from default template arguments, all uses of template parameters in nondeduced contexts are replaced with the corresponding deduced or default argument values. If the substitution results in an invalid type, as described above, type deduction fails.