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

Gavin, thank you again for your time and expertise! Thanks for the reference! One way to verify I was defining overloads is to remove the template function declaration. After removing the declaration, the code compiles, without complaining that the declaration is missing, which is necessary for specialization. On 23.11.2017 06:11, Gavin Lambert via Boost-users wrote:

On 23/11/2017 01:11, Ireneusz Szcześniak wrote:

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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:

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template <typename Graph> auto get_cost(const Label1<Graph> &l) {   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:

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template <typename Graph> auto get_cost(const Label1<Graph> &l) {   return std::get<0>(l); }

but I get (with gcc version 7.2.0):

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error: non-class, non-variable partial specialization ‘get_cost’ is not allowed get_cost(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.

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