ISO/IEC JTC1 SC22 WG21 P0683R0
Jens Maurer <Jens.Maurer@gmx.net>
Target audience: EWG and CWG
2017-06-18

P0683R0: Default member initializers for bit-fields

Introduction

This paper presents wording for the syntax chosen for default member initializers for bit-fields during the WG21 meeting in Issaquah 2016. It is a follow-on paper to P0187R1 "Proposal/Wording for Bit-field Default Member Initializer Syntax" by Andrew Tomazos.

In a nutshell, the proposal with the most consensus was to support 

  struct S {
    int x : 5 = 42;
  };
with the proviso that parsing ambiguities such as 
  int a;
  struct S2 {
    int y : true ? 1 : a = 42;
    int z : 1 || new int { 42 };
  };
are resolved with a max-munch rule.

Discussion

The grammar for the member-declarator of a bit-field (12.2.4 [class.bit]) is 
  identifieropt attribute-specifier-seqopt : constant-expression
In turn, a constant-expression is a conditional-expression (see 8.20 [expr.const]), therefore a = cannot possibly be part of that constant-expression at the top level. This makes the motivating use-case work with a max-munch rule, because parsing of the constant-expression will stop when the = is encountered.

The ambiguous cases shown above are both parsed as convoluted constant-expressions, so neither S2::y nor S2::z have a default member initializer, preserving backward compatibility. The meaning can be changed by applying parentheses: 

  int a;
  struct S2 {
    int y : (true ? 1 : a) = 42;
    int z : (1 || new int) { 42 };
  };

Wording changes

In 12.2 [class.mem] paragraph 1, change 
  member-declarator:
    declarator virt-specifier-seqopt pure-specifieropt
    declarator brace-or-equal-initializeropt
    identifieropt attribute-specifier-seqopt : constant-expression brace-or-equal-initializeropt
Add after 12.2 [class.mem] paragraph 7:
In a member-declarator, an = immediately following the declarator is interpreted as introducing a pure-specifier if the declarator-id has function type, otherwise it is interpreted as introducing a brace-or-equal-initializer. [ Example: ... ]

In a member-declarator for a bit-field, the constant-expression is parsed as the maximum sequence of tokens that could possibly form a conditional-expression. [ Example: 

  int a;
  const int b = 0;
  struct S {
    int x : 5 = 42;                 // ok; "= 42" is brace-or-equal-initializer
    int y1 : true ? 1 : a = 42;     // ok; brace-or-equal-initializer is absent
    int y2 : true ? 1 : b = 42;     // error: cannot assign to const int
    int z : 1 || new int { 42 };    // ok; brace-or-equal-initializer is absent
  };
-- end example ]
In 12.2.4 [class.bit] paragraph 1, change
A member-declarator of the form 
  identifieropt attribute-specifier-seqopt : constant-expression brace-or-equal-initializeropt
specifies a bit-field; its length is set off from the bit-field name by a colon.