Home Original page

example sources in both Python and C++

Daniel Berlin dberlin at redhat.com
Tue Oct 3 00:52:54 EDT 2000 
wmcclain at salamander.com (Bill McClain) writes:

> > >      * Template arguments cannot be local types. That means STL
> > >        containers must use types declared at the outer scope, which
> > >        is ugly.
> > Wait, what?
> > Explain what you mean, this is confusing.
> > If you mean you can only use types in the outermost, or global scope,
> > as template arguments, your compiler is severely broken.
> 
> Try this on your compiler and tell me what happens:
> 
> ***********
> #include <list>
> 
> class Legal {
>     int a;
>     };
> 
> int main() {
>     class Illegal {
>         int a;
>         };
> 
>     list<Legal> ok;         // compiles
>     list<Illegal> nogood;   // does not compile
>     return 0;
>     }
> 
> // g++
> // temp.cpp:13: type `main()::Illegal' composed from a local type is not a valid template-argument
> 
> // VMS
> // %CXX-E-LOCALTYPTMPLARG, a template argument may not reference a local type
> // at line number 13 in file CTS_DEVELOPER:[WMCCLAIN]TEMP.CPP;1
> ***********
> 
> I'm too tired to turn on NT to try VC++.
> 
> -Bill
Correct, this is indeed illegal.
You cannot use function local types as template arguments, because it
would make instantiation very evil.

14.3.1  Template type arguments [temp.arg.type]
1    A template-argument for a template-parameter which is a type shall be a type-id.
2    A local type, a type with no linkage, an unnamed type or a type compounded
from any of these types shall not be used as a template-argument for a
template type-parameter.  


Why would you ever want to use a local type as a template argument?

However, when you say outer scope, you are wrong.
You can use a type declared in any scope, except the local one, as a
template argument.
IE nested classes, nested classes in a different namespace, whatever.
--Dan
More information about the Python-list mailing list