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I am reading about deduction guides in C++17. So say we have the following example:

template<typename T> struct Custom 
{
  
};
template<typename T> struct Person 
{
    Person(Custom<T> const&);
    Person(Custom<T>&&);
};
template<typename T> Person(Custom<T>) -> Person<T>; //explicitly declared deduction guide

My question is that will the explicit declaration for the explicit deduction guide(as shown above) disable the formation of the 2 implicit deduction guides(corresponding to the 2 ctors of class template Person) that would have been there if we did not specify an explicit deduction guide. I mean, suppose in the above example, there was no explicit deduction guide, then there will be 2 implicit deduction guides(corresponding to the two constructors of Person). But after we have added explicit guide, will we have a total of 1 deduction guide(explicitly declared by us the user) or 3 deduction guides(including 2 implicit deduction guides).

PS: Note that the question is only about the deduction guides for Person and not about whether implicit guide will be formed for Custom.

Jason
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1 Answers1

8

No, deduction-guide declarations do not suppress the formation of implicit deduction guides during the class template argument deduction process. The overload set will contain both. See [over.match.class.deduct]/1:

... a set of functions and function templates, called the guides of C, is formed comprising:

  • If C is defined, for each constructor of C, a function template with the following properties: ...
  • If C is not defined or does not declare any constructors, an additional function template derived as above from a hypothetical constructor C().
  • An additional function template derived as above from a hypothetical constructor C(C), called the copy deduction candidate.
  • For each deduction-guide, a function or function template with the following properties: ...

[additional provisions related to aggregates omitted...]

However, the third last tiebreaker in the overload resolution process gives precedence to deduction-guides (i.e., explicitly declared deduction guides) over implicit deduction guides. [over.match.best.general]/2.10. This tiebreaker is only reached after all of the more general tiebreakers (not related directly to deduction guides) are tried.

Brian Bi
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    @AnoopRana If you want to see the numbering, you can click on the links. I am only summarizing the parts that are directly relevant to the answer and I believe that adding numbering to the bullet points will add clutter instead of making the answer easier to read. – Brian Bi May 09 '22 at 16:40
  • So does this means that for class template `Person` we will still have 3 deduction guides in total? One explicitly deduction guide and two corresponding to the ctors of `Person`. Also note that my question is only about the deduction guides for `Person` and not about whether implicit guide will be formed for `Custom`. – Jason May 09 '22 at 16:56
  • I see 2 problems with this answer. 1) Formatting is too bad(which is specific to me). 2) The answer does not clearly answer my question. In particular, i specifically asked will we have 1 or 3 deduction guides for `Person` or maybe more if i am wrong. I recommend updating it accordingly. – Jason May 09 '22 at 17:13
  • @AnoopRana The number of deduction guides for `Person` is a corollary of what is stated in the answer. The first bullet point contributes 2 deduction guides, the third contributes 1, and the fourth contributes 1. – Brian Bi May 09 '22 at 18:04
  • Ok, this comment is what is needed(should be added IMO to) in the answer to make the answer more clear and complete. – Jason May 09 '22 at 18:13
  • @AnoopRana I disagree that it needs to be added to the answer. Like I said, it is a corollary of what is already stated. – Brian Bi May 09 '22 at 18:16
  • It's fine if you don't want to be explicit. I was only suggesting an edition to the answer that would make it more clear. Anyways i got my answer(through the comment that you made). Thanks for the effort and clarification. – Jason May 09 '22 at 18:28