This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

864. braced-init-list in the range-based for statement

[Voted into WP at August, 2010 meeting.]

The intent is that the range-based for statement should be able to be used with a braced-init-list as the range over which to iterate. However, this does not work grammatically: a braced-init-list is not an expression, as required by the syntax in 8.6.5 [stmt.ranged] paragraph 1:

for ( for-range-declaration : expression ) statement

Even if this were resolved, the “equivalent to” code is not correct. It contains the declaration,

auto && __range = ( expression );

This has a similar problem, in that 9.2.9.7 [dcl.spec.auto] paragraph 3 requires that the initializer have one of the forms

= assignment-expression
( assignment-expression )

which does not allow for a braced-initializer-list. In addition, although not allowed by the grammar, 9.2.9.7 [dcl.spec.auto] paragraph 6 treats the braced-init-list specially, in order for the type deduction to work correctly:

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 (9.4.5 [dcl.init.list]), with std::initializer_list<U>.

The problem here is that a parenthesized initializer, as in the code expansion of the range-based for statement, is not a braced-init-list.

Proposed resolution (June, 2010):

1. Change 8.6 [stmt.iter] paragraph 1 as follows:

Iteration statements specify looping.

iteration-statement:
while ( condition ) statement
do statement while ( expression ) ;
for ( for-init-statement condition_opt ; expression_opt ) statement
for ( for-range-declaration : expression for-range-initializer ) statement




for-init-statement:
expression-statement
simple-declaration




for-range-declaration:
type-specifier-seq attribute-specifier_opt declarator




for-range-initializer:
expression
braced-init-list

[Note: a for-init-statement ends with a semicolon. —end note]

2. Change 8.6.5 [stmt.ranged] paragraph 1 as follows:

The For a range-based for statement of the form

for ( for-range-declaration : expression ) statement

let range-init be equivalent to the expression surrounded by parentheses:

( expression )

[Footnote: this ensures that a top-level comma operator cannot be reinterpreted as a delimiter between init-declarators in the declaration of __range. —end footnote] and for a range-based for statement of the form

for ( for-range-declaration : braced-init-list ) statement

let range-init be equivalent to the braced-init-list. In each case, a range-based for statement is equivalent to

  {
    auto && __range = ( expression ) range-init;
    for ( auto __begin = begin-expr,
    ...

Note to editor:

The formatting in the preceding change for range-init follows that of the existing text for begin-expr and end-expr. However, CWG is concerned that this style makes all of these elements look too much like grammar nonterminals and asks that the editor consider some other formatting convention.