"Transparency" of a template template argument (missing template argument)

Question

To put it somewhat vaguely, from the snippet (tt.cc) below it seems to me as if the template template notion lacked some kind of a "transparency". While I don't seem to fathom what other template argument should A::f() be provided (besides the template template one), I might as well be just missing some banality here.

template <class T>
class A {
public:
  template< template<class> class U >
  void f( int i ) {
    // ...
  }
};


template< class T, template<class> class U >
class B {
public:
  void f( int i ) {
    A<T> a;
    a.f<U>( i );
  }
};


int main() {
  return 0;
}

The error message it provides is:

g++ ./tt.cc -o tt
./tt.cc: In member function ‘void B<T, U>::f(int)’:
./tt.cc:17:10: error: missing template arguments before ‘>’ token
     a.f<U>( i );
          ^
Compilation exited abnormally with code 1

Any ideas greatly appreciated.

@0x499602D2 I always pity the users who experience this because it's hard to google for it if you don't know the answer — TemplateRex, Mar 06 '14 at 22:13
@TemplateRex That's also been discussed [here](http://meta.stackexchange.com/questions/167123/question-closed-as-duplicate-could-not-be-found-by-the-op). — David G, Mar 06 '14 at 22:15

score 3 · Accepted Answer · edited May 23 '17 at 11:59

3

What you are seeing is the compiler interpreting the > token inside f<U> as the inequality operator. You need to add .template to let the compiler know you mean a template argument.

  void f( int i ) {
    A<T> a;
    a.template f<U>( i );
  }

Live Example.

See also Where and why do I have to put the “template” and “typename” keywords?.

edited May 23 '17 at 11:59

Community

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answered Mar 06 '14 at 22:10

TemplateRex

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thank you! this operator misinterpretation is kind of an eye-opener... :-) – calmity Mar 06 '14 at 22:23
@calmity especially because the Visual C++ compiler lets you get away with it in many instances – TemplateRex Mar 06 '14 at 22:24

"Transparency" of a template template argument (missing template argument)

1 Answers1