Annex C (informative) Compatibility [diff]

Affected subclause: [class.copy]
Change: Copying volatile objects.

The implicitly-declared copy constructor and implicitly-declared copy assignment operator cannot make a copy of a volatile lvalue.

For example, the following is valid in ISO C:

struct X { int i; };
volatile struct X x1 = {0};
struct X x2 = x1;               // invalid C++
struct X x3;
x3 = x1;                        // also invalid C++

Rationale: Several alternatives were debated at length.

Changing the parameter to volatile const X& would greatly complicate the generation of efficient code for class objects.

Discussion of providing two alternative signatures for these implicitly-defined operations raised unanswered concerns about creating ambiguities and complicating the rules that specify the formation of these operators according to the bases and members.

Effect on original feature: Deletion of semantically well-defined feature.

Difficulty of converting: Semantic transformation.

If volatile semantics are required for the copy, a user-declared constructor or assignment must be provided.

If non-volatile semantics are required, an explicit const_­cast can be used.

How widely used: Seldom.