[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.