Using templates with anonymous classes scoped inside a function

Question

Let's say I have the following snippet:

template <class T> void f(T arg) { arg(); }

void g()
{
   struct { void operator()(void) { } } foo;

   f(foo);
}

Visual C++ accepts this. However, when I try GCC, I get:

$ g++ --version # just in case this matters
g++ (Debian 4.4.5-8) 4.4.5
...
$ g++ foo.cc
foo.cc: In function 'void g()':
foo.cc:7: error: no matching function for call to 'f(g()::<anonymous struct>&)'

When foo is scoped globally and its type has a name, this works. But when the type is anonymous or declared inside g() it does not.

Why does GCC reject this? Is it valid C++?

score 7 · Accepted Answer · answered Jan 11 '11 at 15:25

7

14.3.1 paragraph 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 templateargument for a template typeparameter.

In other words, not valid. Although it would be handy imo, that's maybe why VC allows it.

answered Jan 11 '11 at 15:25

stijn

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frustrating, indeed. This would have been a workable alternative to the lack of closures in C++ for instance. – Alexandre C. Jan 11 '11 at 15:27
1

Thanks for such a straightforward answer. To me it seems like this usage fits the "spirit" of the language, but I can also see why this might have been scoped out to save implementers some hassle. – asveikau Jan 11 '11 at 15:53
1

exactly. It's a pitty one has to clutter the global or other namespaces with structs that don't really belong there but just get used once in a single function.. – stijn Jan 11 '11 at 15:56
1

you can always use unnamed-namespaces for those kind of functors! – ltjax Jan 11 '11 at 16:34

score 2 · Answer 2 · answered Jan 11 '11 at 15:58

2

As already said, a local class (a class defined within a function) can not be used as a template argument. Fortunately, C++0x fixes that with lambda functions: http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions

answered Jan 11 '11 at 15:58

Maxim Egorushkin

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I think C++0x even allows local-classes to be used as template arguments! – ltjax Jan 11 '11 at 16:32

Using templates with anonymous classes scoped inside a function

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