c++11 : Local and Unnamed Types as Template Arguments

In N2402, Anthony Williams proposes that local types, and unnamed types be usable as template arguments. At the February 2008 (Bellevue) meeting, the Core working group supported the use of local types but was concerned about unnamed types. In addition, the WG did not support the mechanism used to accomplish this change in N2402.

I discussed the unnamed type issue with Anthony Williams and others. Use of unnamed types as template arguments was considered a useful facility and does not seem to present any particular implementation issues. In particular, it is fairly common practice to use unnamed enumerations in header files. Consequently, unnamed types have been retained in this proposal. Note that a namespace scope unnamed enumeration in one translation unit is distinct from one in another translation unit. So in the example below, x and y are initalized by two different instances of f

file1.c: template <class T> int f(T t); enum {e1}; int x = f(e1); file2.c: template <class T> int f(T t); enum {e1}; int y = f(e1);

N2402 makes local types and unnamed types usable as template arguments by giving them linkage. As mentioned above, the Core Working Group did not favor this solution. This document provides drafting to allow local and unnamed types to be used as template arguments by revising the rules for template arguments to permit such types.

Working Paper Changes

Change 3.5 [basic.link] paragraph 8 as follows:

A type without linkage shall not be used as the type of a variable or function with linkage, unless

• the variable or function has extern "C" linkage., or
• the type without linkage was named using a dependent type (14.6.2.1).

Delete the following sentence from 3.5 [basic.link] paragraph 8:

This implies that names with no linkage cannot be used as template arguments (14.3).

Add the following example after paragraph 8:

[ Example: template <class T> struct A { // in A<X>, the following is allowed because the type with no linkage // X is named using template parameter T. friend void f(A, T){} }; template <class T> void g(T t) { A<T> at; f(at, t); } int main() { class X {} x; g(x); } -- end example]

In 14.3.1 [temp.arg.type] paragraph 2 remove the following text:

A type without linkage (3.5) shall not be used as a template argument for a template type parameter.

Replace the example in paragraph 2 with the following:

[ Example: template <class T> class X { }; template <class T> void f(T t) { } struct {} unnamed_obj; void f() { struct A { }; enum { e1 }; typedef struct {} B; B b; X<A> x1; // OK X<A*> x2; // OK X<B> x3; // OK f(e1); // OK f(unnamed_obj); // OK f(b); // OK } -- end example]

End of document.