How to define deduction guide with templated constructor?

Question

In order to simplify my problem, I'll be using std::unique_lock as the tool to explain. std::unique_lock has a template argument, the mutex. However, it's constructor is also a template function unique_lock(TMutex &, const chrono::duration<_Rep, _Period>&).

When one uses this, one can write:

 auto lock = std::unique_lock(my_mutex, 5s);

So, the question: How to write out the deduction guide for this (without changing behavior), how to do so?

My best attempt upto now:

template<typename _Mutex>
template<typename _Rep, typename _Period>
unique_lock(_Mutex &, const chrono::duration<_Rep, _Period>&) -> unique_lock<_Mutex>;

Unfortunately, clang doesn't accept this:

error: extraneous template parameter list in template specialization or out-of-line template definition

Have you tried just a single `template`, with three template parameters? — Sam Varshavchik, Aug 04 '19 at 20:20
Not yet, it that works, I'm gonna be confused on why that would be the right way of writing this — JVApen, Aug 04 '19 at 20:21
Looks like that was the trick. Why did they make that inconsistent with defining a function? — JVApen, Aug 04 '19 at 20:24
@JVApen It deduces `_Mutex` from the argument type. For example, what if you had `unique_lock>` instead? — Artyer, Aug 04 '19 at 20:24
@Artyer I'm pretty convinced that should work and I would write it in a way that it explicitly deduces as you suggest.,However, I didn't design `std::unique_lock` and I expect it might misbehave if you move the mutex. (However, as that's not a good idea, I'm not that worried about it) — JVApen, Aug 04 '19 at 20:29
As for defining a function outside of the class definition, that's the way you write it — JVApen, Aug 04 '19 at 20:32
@JVApen For template member functions of template classes, the first template is deduced from `this`. Since there is no `this`, there would be no way to deduce that separately from the arguments so it doesn't make sense — Artyer, Aug 04 '19 at 20:52
Forgive me if I'm wrong, but aren't `_Names` beginning with underscore followed by a capital letter reserved? — Fureeish, Aug 04 '19 at 21:03
@Fureeish: they are, I explicitly used an example from the standard library as example for familiarity. I've copied the Ctor from MSVCs implementation — JVApen, Aug 04 '19 at 21:12

Joel Filho · Accepted Answer · 2019-08-04T21:00:20.150

GCC has a better error message for this:

error: too many template-parameter-lists

You can change it to a single template parameter list, like this:

template<typename _Mutex, typename _Rep, typename _Period>
unique_lock(_Mutex &, const chrono::duration<_Rep, _Period>&) -> unique_lock<_Mutex>;

And it works.

From the comments in your question, you seem to be mixing CTAD and specializations.

You're not specializing anything in unique_lock. Not a member function, not a constructor, you're just defining a deduction guide. More specifically, from cppreference:

The syntax of a user-defined deduction guide is the syntax of a function declaration with a trailing return type [...] A deduction guide is not a function [...]

Notice that it has a syntax of a declaration, not a specialization. It's just different from what you expected.

How to define deduction guide with templated constructor?

1 Answers1