New Rules for auto deduction from braced-init-list

ISO/IEC JTC1 SC22 WG21 N3922 - 2014-02-13

Synopsis

This paper provides proposed wording to change the rules for auto deduction from a braced-init-list. In particular, the rules for deduction of init-captures from braced-init-list are modified.

Background

For background information see N3681, "Auto and braced-init-lists", by Voutilainen, and N3912, "Auto and braced-init-lists, continued", also by Voutilainen.

In Issaquah, EWG considered two alternative resolutions; this paper offers wording for one of the two, and mentions the other only as an acceptable alternative in case CWG uncovers fatal problems with EWG's preferred resolution.

Direction from EWG is that we consider this a defect in C++14.

Summary of Proposed Changes

For copy-list-initialization, auto deduction will either deduce a std::initializer_list (if the types of entries in the braced-init-list are all identical) or be ill-formed otherwise.

For direct list-initialization:

1. For a braced-init-list with only a single element, auto deduction will deduce from that entry;
2. For a braced-init-list with more than one element, auto deduction will be ill-formed.

The rules for range-based for are unaffected by the changes proposed here, as the rules for copy-list-initialization are unmodified.

Note: C++14 already disallows deduction of an function return type from a braced-init-list, and that's a good thing.

Alternative

There was also support (though less strong) in EWG for an alternative proposal in which auto deduction does not distinguish between direct list-initialization and copy list-initialization, and which allows auto deduction from a braced-init-list only in the case where the braced-init-list consists of a single item. Possible wording was for that alternative was included in N3681 by Voutilainen.

Wording

Modify 7.1.6.4p7 [dcl.spec.auto] as shown below:

When a variable declared using a placeholder type is initialized, or a return statement occurs in a function declared with a return type that contains a placeholder type, the deduced return type or variable type is determined from the type of its initializer. In the case of a return with no operand, the initializer is considered to be void(). Let T be the declared type of the variable or return type of the function. If the placeholder is the auto type-specifier the deduced type is determined using the rules for template argument deduction. If the initialization is direct-list-initialization then the braced-init-list shall contain only a single initializer-clause L. If the deduction is for a return statement and the initializer is a braced-init-list (8.5.4), the program is ill-formed. Otherwise, obtain P from T by replacing the occurrences of auto with either a new invented type template parameter U or, if the initializer is a braced-init-list, the initialization is copy-list-initialization, with std::initializer_list<U>. Deduce a value for U using the rules of template argument deduction from a function call (14.8.2.1), where P is a function template parameter type and the initializer is the corresponding argument. the corresponding argument is the initializer, or L in the case of direct-list-initialization. If the deduction fails, the declaration is ill-formed. Otherwise, the type deduced for the variable or return type is obtained by substituting the deduced U into P.

and

[Example:

auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int>
auto x2 = { 1, 2.0 }; // error: cannot deduce element type
auto x3{ 1, 2 }; // error: not a single element
auto x4 = { 3 }; // decltype(x4) is std::initializer_list<int>
auto x5{ 3 }; // decltype(x5) is int

Acknowledgements

Thanks to Daveed Vandevoorde and Richard Smith for reviewing this paper, and to Ville Voutilainen for writing the papers preceding this one.