Templated function on templated subclass

Question

I'm having a bit of trouble trying to figure out why gcc cannot deduce the template arguments in the following code:

template <int N>
struct A {
    template <int M>
    struct B {
    };
};

template <int N, int M>
void function(typename A<N>::template B<M> &b) {
    // do stuff
}

int main() {
    A<1>::B<2> b;
    function(b);
}

Error:

$ g++ -std=c++11 -std=gnu++11 test.cpp
test.cpp: In function ‘int main()’:
test.cpp:19:12: error: no matching function for call to ‘function(A<1>::B<2>&)’
  function(b);
            ^
test.cpp:19:12: note: candidate is:
test.cpp:13:6: note: template<int N, int M> void function(typename A<N>::B<M>&)
 void function(typename A<N>::template B<M> &b) {
      ^
test.cpp:13:6: note:   template argument deduction/substitution failed:
test.cpp:19:12: note:   couldn't deduce template parameter ‘N’
  function(b);

Are you sure you're compiling with the c++11 option enabled? — littleadv, May 04 '14 at 21:25
A<1>::B<2> b(); -- I am always a little unsure about this, but isnt this the most vexing parse? — Philipp Lenk, May 04 '14 at 21:25
I have edited the question and placed the error in the question body. — , May 04 '14 at 21:25
@PhilippLenk: It's rapidly becoming the mostly parsed vex by now. — Kerrek SB, May 04 '14 at 21:28
Why the downvote, his question is legit and I think @KerrekSB is correct. — Janick Bernet, May 04 '14 at 21:29
That was a mistake I made while transferring the code; it is not a function in the code that was executed (fixed in question body). — , May 04 '14 at 21:30
Here is what you are looking for: http://stackoverflow.com/questions/12640808/nested-template-and-parameter-deducing — vsoftco, May 04 '14 at 21:37

score 0 · Answer 1 · edited May 23 '17 at 12:11

0

I leave this answer mainly because of the comments. The question has apparently been previously posted and answered.

But as a rule of thumb, for template deduction, in my experience use clang as replacement/alternative, it has much better error messages for such issues (like why template deduction failed).

edited May 23 '17 at 12:11

Community

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answered May 04 '14 at 21:29

Janick Bernet

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This was a mistake I made while transferring the code. The actual code that was run to produce the error output is "correct" in this sense. – May 04 '14 at 21:31
1

Ok, still I would suggest using clang to get a 'second opinion' so to speak. – Janick Bernet May 04 '14 at 21:32
Thanks, downloading right now – May 04 '14 at 21:33
The clang output was pretty much the same; it said it could not infer the template parameter – May 04 '14 at 21:35
3

This link explains why you cannot do exactly what you want: http://stackoverflow.com/questions/12640808/nested-template-and-parameter-deducing – vsoftco May 04 '14 at 21:40

Templated function on templated subclass

1 Answers1