Re: [Boost-users] function template argument deduction with std::tuple

Subject: Re: [Boost-users] function template argument deduction with std::tuple
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-11-23 05:11:01

On 23/11/2017 01:11, Ireneusz Szcześniak wrote:
> However, it bugs me that I'm still not sure whether I've implemented a 
> function-template specialization or a template for function 
> overloading.  You claim that this is an overload:
> 
>> template <typename Graph>
>> auto
>> get_cost(const Label1<Graph> &l)
>> {
>> Â  return std::get<0>(l);
>> }
> 
> Could you please elaborate on how to tell one from the other?  Had it 
> been a specialization, a compiler should accept the explicit template 
> argument:
> 
>> template <typename Graph>
>> auto
>> get_cost<Label1<Graph>>(const Label1<Graph> &l)
>> {
>> Â  return std::get<0>(l);
>> }
> 
> but I get (with gcc version 7.2.0):
> 
>> error: non-class, non-variable partial specialization
>> ‘get_cost<Label1<Graph> >’ is not allowed
>> get_cost<Label1<Graph>>(const Label1<Graph> &l)
> 
> So it seems that I was defining overloads (these don't have template 
> arguments), not specializations as I thought.  The C++ Programming 
> Language, 4th edition, the bottom of page 737, says that you can drop 
> the explicit template argument in the definition of a template 
> specialization, if it can be deduced.  So I dropped the explicit 
> template argument, and still considered the definition the specialization.
> 
> I have a hard time understanding the above error message, thought.  I 
> understand that "non-class" simply means that it's not a member 
> function, OK.  But what "non-variable partial specialization" could 
> possibly mean?  What is "non-variable" here, and why "partial 
> specialization" when I gave all (i.e., one) template argument.  I 
> consider it a complete specialization albeit dependent on a template 
> parameter Graph.

I suggest reading http://www.gotw.ca/publications/mill17.htm.

The short version is that function templates are a bit weird, and if you 
have anything other than template<> it's an overload, because they don't 
do partial specialization.