Template parameter initialization
13.4.3 [temp.arg.nontype]
Davis Herring

Created on 2020-09-21.00:00:00 last changed 6 months ago


Date: 2020-09-21.00:00:00

The initialization of template parameters is severely underspecified. The only descriptions in the existing wording that apply are that the argument is “[converted] to the type of the template-parameter” (13.6 [temp.type] bullet 1.3) and, in 13.4.3 [temp.arg.nontype] paragraph 2,

A template-argument for a non-type template-parameter shall be a converted constant expression (7.7 [expr.const]) of the type of the template-parameter.

This omission is particularly important for template parameters of class type with lvalue template parameter objects whose addresses can be examined during construction. See also issue 2450.

Suggested resolution:

To avoid address-based paradoxes, template arguments for a template parameter of class type C that are not of that type or a derived type are converted to C to produce an exemplar. No restrictions are imposed on the conversion from a template argument to a constructor parameter, since explicit and list-initialization may already be used to limit conversions in a similar fashion. Template arguments that are of such a type are used directly as the exemplar (potentially after a materialization conversion); the effect is as if the template parameter were of type const C& (except that temporaries are allowed). (In the latter case, we must impose some restrictions on glvalue template parameters to interpret them.) Each exemplar is used to copy-initialize the template parameter object to which it is (to be) template-argument-equivalent; the initialization is required to produce a template-argument-equivalent value. The multiple initializations of the template parameter object are (required to be) all equivalent and produce no side effects, so it is unobservable which happen.

Date User Action Args
2020-09-21 00:00:00admincreate