6.7.1 Nonlocal Name Resolution and Instantiation

Sometimes a template definition uses names that are not defined by the template arguments or within the template itself. If so, the compiler resolves the name from the scope enclosing the template, which could be the context at the point of definition, or at the point of instantiation. A name can have different meanings in different places, yielding different resolutions.

Name resolution is complex. Consequently, you should not rely on nonlocal names, except those provided in a pervasive global environment. That is, use only nonlocal names that are declared and defined the same way everywhere. In the following example, the template function converter uses the nonlocal names intermediary and temporary. These names have different definitions in use1.cc and use2.cc, and will probably yield different results under different compilers. For templates to work reliably, all nonlocal names (intermediary and temporary in this case) must have the same definition everywhere.

use_common.h
// Common template definition
template <class Source, class Target>
Target converter(Source source)
       {temporary = (intermediary)source;
       return (Target)temporary;}
use1.cc
typedef int intermediary;
int temporary;

#include "use_common.h"
use2.cc
typedef double intermediary;
unsigned int temporary;

#include "use_common.h"

A common use of nonlocal names is the use of the cin and cout streams within a template. Few programmers really want to pass the stream as a template parameter, so they refer to a global variable. However, cin and cout must have the same definition everywhere.