Movatterモバイル変換

Proposed resolution (January, 2019):

1. Change 7.2.1 [basic.lval] paragraph 11 as follows:

If a program attempts to access the stored value of anobject through a glvalueof other thanwhose type is not similar (7.3.6 [conv.qual])to one of the following types the behavior isundefined:⁵⁸

• the dynamic type of the object,

• a cv-qualified version of the dynamic type of the object,

• a type similar (as defined in 7.3.6 [conv.qual]) tothe dynamic type of the object,

• a type that is the signed or unsigned typecorresponding to the dynamic type of the object,

• a type that is the signed or unsigned typecorresponding to a cv-qualified version of the dynamic typeof the object,

• an aggregate or union type that includes one of theaforementioned types among its elements or non-static datamembers (including, recursively, an element or non-staticdata member of a subaggregate or contained union),

• a type that is a (possibly cv-qualified) base class type ofthe dynamic type of the object,

• achar,unsigned char, orstd::byte type.

If a program invokes a defaulted copy/move constructoror copy/move assignment operator for a union of typeUwith a glvalue argument that does not denote an object of typecvU within its lifetime, the behavior isundefined. [Note: Unlike in C, C++ has no accesses ofclass type. —end note]

2. Change 7.3.6 [conv.qual] paragraph 1 as follows:

Acv-decomposition of a typeTis a sequence ofcv_iandP_i such thatT is

“cv₀ P₀ ··· cv_n-1 P_n-1 cv_nU” forn>≥ 0,

where...

Additional note, October, 2015:

It has been suggested that the aliasing rules should be extended to permit an object ofan enumeration with a fixed underlying type to alias an object withthat underlying type.

Suggested resolution:

Replace 7.2.1 [basic.lval] paragraph 10 as follows:

If a program attempts to access the stored value of an objectthrough a glvalue whose type is not similar (7.3.6 [conv.qual])to one of the following types the behavior is undefined:[Footnote:... —end footnote]

• the dynamic type of the object,

• the signed or unsigned type corresponding to the dynamictype of the object, or

• achar orunsigned char type.

[Accepted as a DR at the February, 2019 meeting.]

The aliasing rules of 7.2.1 [basic.lval] paragraph 10 wereadapted from C with additions for C++. However, a number of the pointseither do not apply or are subsumed by other points. For example, theprovision for aggregate and union types is needed in C for structassignment, which in C++ is done via constructors and assignmentoperators in C++, not by accessing the complete object.