Document: N1409
Date: 2009/09/29

Defect Report #nnn

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Submitter: Richard Hansen (BBN Technologies)
Submission Date: 2007-08-04
Source: Randy Meyers<USA>
Reference Document: N/A
Date: 2007-08-04
Subject: aliasing and effective type as it applies to unions/aggregates


My question concerns the phrasing of bullet 5 of 6.5p7 (aliasing as it applies to unions/aggregates). Unless my understanding of effective type is incorrect, it seems like the union/aggregate condition should apply to the effective type, not the lvalue type.

Here are some more details:

Take the following code snippet as an example:

union {int a; double b;} u;
u.a = 5;

From my understanding of the definition of effective type (6.5p6), the effective type of the object at location &u is union {int a; double b;}. The type of the lvalue expression that is accessing the object at &u (in the second line) is int.

From my understanding of the definition of compatible type (6.2.7), int is not compatible with union {int a; double b;}, so bullets 1 and 2 of 6.5p7 do not apply. int is not the signed or unsigned type of the union type, so bullets 3 and 4 do not apply. int is not a character type, so bullet 6 does not apply.

That leaves bullet 5. However, int is not an aggregate or union type, so that bullet also does not apply. That means that the above code violates the aliasing rule, which it obviously should not.

I believe that bullet 5 should be rephrased to indicate that if the effective type (not the lvalue type) is an aggregate or union type that contains a member with type compatible with the lvalue type, then the object may be accessed.

Suggested Technical Corrigendum

Here is a possible rewrite of 6.5p7:

An object with effective type Q shall have its stored value accessed only by an lvalue expression of type T when one of the following applies:

Committee Discussion

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