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Brent Sullivan
Brent Sullivan

The Deduction Guide !!EXCLUSIVE!!


Similar to the template specialization, the deduction guide should also be declared within the same semantic scope as the class template. So why can I specialize the class template outside the inline namespace, but for a deduction guide I can't?




The Deduction Guide


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Deduction guides are not like template specializations. They are considered part of the class template's definition, much like constructors and other member functions. As such, they are expected to be written by the creator of the class, typically immediately following the template class's definition. Given these expectations, it doesn't make sense for deduction guides to exist in a scope other than the scope of the template class definition itself.


The very first version of the CTAD proposal, as well as every derivative version thereof, focuses on mapping constructor arguments to class template parameters. What will eventually be known as "deduction guides" were first discussed as "Canonical factory functions". But the text around it is particularly telling:


Notice how focused the text is on "constructors". These canonical factory functions are maps between constructors and template arguments. They are considered, conceptually at least, to be constructors of a sort. After all, implicit guides are generated from constructors, so it stands to reason that explicit guides are conceptually equivalent to a class constructor.


Indeed, the prototypical example of why you need explicit deduction guides (that is, why you can't rely entirely on implicit guides) is focused on the constructors of the type. Namely, vector's iterator constructor:


The bottom line is this: explicit deduction guides are built around a class's constructors (though those constructors don't have to exist). They exist to map constructor argument types to class template parameters. If you cannot add a constructor to a class from outside of the class's definition, then it stands to reason that you cannot add an explicit deduction guide from outside of the class's definition either.


The C++17 standard introduces "template deduction guides". I gather they're something to do with the new template argument deduction for constructors introduced in this version of the standard, but I haven't yet seen a simple, FAQ-style explanation of what they are and what they're for.


Template deduction guides are patterns associated with a template class that tell the compiler how to translate a set of constructor arguments (and their types) into template parameters for the class.


You need guides when the deduction of the type from the arguments is not based on the type of one of those arguments. Initializing a vector from an initializer_list explicitly uses the vector's T, so it doesn't need a guide.


The left side doesn't necessarily specify an actual constructor. The way it works is that, if you use template constructor deduction on a type, it matches the arguments you pass against all deduction guides (actual constructors of the primary template provide implicit guides). If there is a match, it uses that to determine which template arguments to provide to the type.


But once that deduction is done, once the compiler figures out the template parameters for the type, initialization for the object of that type proceeds as if none of that happened. That is, the deduction guide selected does not have to match the constructor selected.


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In the current Working Draft, an aggregate class is defined as a class with no user-declared or inherited constructors, no private or protected non-static data members, no virtual functions, and no virtual, private or protected base classes. While we would like to generate an aggregate deduction guide for class templates that comply with these rules, we first need to consider the case where there is a dependent base class that may have virtual functions, which would violate the rules for aggregates. Fortunately, that case does not cause a problem because any deduction guides that require one or more arguments will result in a compile-time error after instantiation, and non-aggregate classes without user-declared or inherited constructors generate a zero-argument deduction guide anyway. Based on the above, we can safely generate an aggregate deduction guide for class templates that comply with aggregate rules.


When P0960R0 was discussed in Rapperswil, it was voted that in order to allow aggregate initialization from a parenthesized list of arguments, aggregate initialization should proceed as if there was a synthesized constructor. We can use the same approach to also synthesize the required additional deduction guide during class template argument deduction as follows:


While this is to be expected, one suspects that had class template argument deduction for alias templates been around when pmr::vector was being designed, the committee would have considered allowing such an inference as safe and useful in this context. If that was desired, it could easily have been achieved by indicating that the pmr::allocator template parameter should be considered non-deducible:


In addition, if C satisfies the conditions for an aggregate class with the assumption that any dependent base class has no virtual functions and no virtual base classes, let e1, ... en be the elements of C ([dcl.init.aggr]). If there is at least one such element, and the initializer is a non-empty braced-init-list or parenthesized expression-list, the set contains an additional function template, called the aggregate deduction candidate, defined as follows. Let x1, ..., xm be the elements of the initializer-list, designated-initializer-list, or expression-list. The aggregate deduction candidate is derived as above from a hypothetical constructor C(T1, ..., Tm), where Ti is the declared type of the element ei of C which would be explicitly initialized by xi as described in [dcl.init.aggr]. In addition, if C inherits constructors (namespace.udecl 9.8) from a base class denoted by a simple-template-id B, the set contains the functions and function templates formed from an alias template (over.match.alias.deduct) whose template parameters are those of C and whose simple-template-id is B.


The left side doesn't necessarily specify a constructor. The way it works is that, if you use template constructor deduction on a type, it matches the arguments you pass against all deduction guides (actual constructors provide implicit guides). If there is a match, it uses that to determine which template arguments to provide to the type. But overload resolution to determine which constructor to call happens after that.


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