Querying the alignment of an object

Motivation

The alignas specifier is applicable to objects, affecting their alignment requirement but not their type. It is therefore currently not possible to determine an object's actual alignment requirement. This paper proposes to permit application of alignof to objects and references.

Proposed wording

When applied to a type, the result of the alignof operator reflects the alignment requirement of the type in the complete-object case.

In some contexts, unevaluated operands appear (5.2.8, 5.3.3, 5.3.6, 5.3.7, 7.1.6.2).

unary-expression:

alignof ( type-id )

alignof postfix-expression

1. An alignof expression yields the alignment requirement of its operand type. If the operand is a type, it shall representrepresenting a complete object type, or an array thereof, or a reference to one of those types. Otherwise, the operand shall be a possibly parenthesized postfix-expression that is an id-expression or class member access (5.2.5 [expr.ref]). The postfix-expression shall designate a variable or class data member. The expression is an unevaluated operand (clause 5 [expr]).
2. The result is an integral constant of type std::size_t.
3. When alignof is applied to a reference type, the result is the alignment of the referenced type. When alignof is applied to an array type, the result is the alignment of the element type.
4. When the operand is a postfix-expression e, the result is determined as follows:
   • If e designates an entity of object type, and any preceding declarations of that entity have an alignment-specifier (7.6.2 [dcl.align]), the result is the alignment specified by those declarations. [Note: All declarations of an entity that have an alignment-specifier must specify equivalent alignment; see 7.6.2 [dcl.align]. —end note]
   • Otherwise, the result is alignof(decltype(e)).
   [Note: As is the case with noexcept, subsequent applications of alignof to the same entity may yield different values. — end note]

If the defining any declaration of an entity has an alignment-specifier, any non-defining all declarations of that entity shall either specify equivalent alignment or have no alignment-specifier. Conversely, if any declaration of an entity has an alignment-specifier, every defining declaration of that entity shall specify an equivalent alignment.No diagnostic is required if declarations of an entity have different alignment-specifiers in different translation units. [Example: … —end example] [Example:

alignas(16) extern int i;

alignas(8) extern int i;      // error: Different alignment specified

extern int i;                 // Ok: No alignment-specifiers

—end example]

Expressions of the following forms are never type-dependent (because the type of the expression cannot be dependent):

[…]

alignof ( type-id )

alignof postfix-expression

[…]

Expressions of the following form are value-dependent:

[…]

alignof postfix-expression

Acknowledgments

References

[EWG98] “[tiny] Core issue 1008, Querying the alignment of an object“: wg21.link/ewg98

[CWG1617] “alignas and non-defining declarations“: wg21.link/1617