Hello all.
I've just read the paper titled "Range-based for statements and ADL"
by Jonathan Wakely and Bjarne Stroustrup at:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3257.pdf
The point of this article is that the argument dependent lookup in the
range-for specification will make some trouble in the real world. E.g:
#include <vector>
namespace n
{
struct X { void begin(); };
struct Y { void begin(); };
template<typename T> void begin(T& t) { t.begin(); }
}
int main()
{
std::vector<n::X> v;
for (auto i : v) // error: call to begin is ambiguous: std::begin
or n::begin?
{
//...
}
}
I've implemented Option 5 from the paper (which is the coolest, IMHO):
- [...], if _rangeT has a member begin or a member end, begin-expr
and end-expr are __range.begin() and __range.end(), respectively
- otherwise, begin-expr and end-expr are begin(__range) and
end(__range), respectively, where begin and end are looked up with
argument-dependent lookup [...]
I'm not sure about what should happen if _rangeT has a member begin
but not a member end. I think that there are just two sensible
options:
* Use them only if both members, begin and end, exist.
* Look for them independently. This is what my patch does.
Also, if the member begin/end is not accessible or not callable, a
compiler error will follow immediately (this is as expected).
I'll appreciate any comments.
Best regards
--
Rodrigo
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index a9fd201..8fc77a0 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -1871,6 +1871,8 @@ static tree cp_parser_c_for
(cp_parser *, tree, tree);
static tree cp_parser_range_for
(cp_parser *, tree, tree, tree);
+static tree cp_parser_perform_range_for_lookup
+ (const char *, tree);
static tree cp_parser_jump_statement
(cp_parser *);
static void cp_parser_declaration_statement
@@ -8870,25 +8872,8 @@ cp_convert_range_for (tree statement, tree range_decl,
tree range_expr)
else
{
/* If it is not an array, we must call begin(__range)/end__range() */
- VEC(tree,gc) *vec;
-
- begin_expr = get_identifier ("begin");
- vec = make_tree_vector ();
- VEC_safe_push (tree, gc, vec, range_temp);
- begin_expr = perform_koenig_lookup (begin_expr, vec,
- /*include_std=*/true);
- begin_expr = finish_call_expr (begin_expr, &vec, false, true,
- tf_warning_or_error);
- release_tree_vector (vec);
-
- end_expr = get_identifier ("end");
- vec = make_tree_vector ();
- VEC_safe_push (tree, gc, vec, range_temp);
- end_expr = perform_koenig_lookup (end_expr, vec,
- /*include_std=*/true);
- end_expr = finish_call_expr (end_expr, &vec, false, true,
- tf_warning_or_error);
- release_tree_vector (vec);
+ begin_expr = cp_parser_perform_range_for_lookup("begin", range_temp);
+ end_expr = cp_parser_perform_range_for_lookup("end", range_temp);
/* The unqualified type of the __begin and __end temporaries should
* be the same as required by the multiple auto declaration */
@@ -8940,6 +8925,42 @@ cp_convert_range_for (tree statement, tree range_decl,
tree range_expr)
return statement;
}
+static tree
+cp_parser_perform_range_for_lookup (const char *name, tree range)
+{
+ tree ident, expr;
+
+ ident = get_identifier (name);
+ expr = build_qualified_name (/*type=*/NULL_TREE,
+ TREE_TYPE (range),
+ ident,
+ /*template_p=*/false);
+ expr = finish_class_member_access_expr(range, expr, false, tf_none);
+
+ if (expr != error_mark_node)
+ {
+ tree instance, fn;
+ instance = TREE_OPERAND (expr, 0);
+ fn = TREE_OPERAND (expr, 1);
+
+ expr = build_new_method_call (instance, fn, NULL, NULL, LOOKUP_NORMAL,
+ NULL, tf_warning_or_error);
+ }
+ else
+ {
+ VEC(tree,gc) *vec;
+ vec = make_tree_vector ();
+ VEC_safe_push (tree, gc, vec, range);
+ expr = get_identifier (name);
+ expr = perform_koenig_lookup (expr, vec,
+ /*include_std=*/true);
+ expr = finish_call_expr (expr, &vec, false, true,
+ tf_warning_or_error);
+ release_tree_vector (vec);
+ }
+ return expr;
+}
+
/* Parse an iteration-statement.
