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Classes not using pointer members pointing to memory controlled by its
objects (and not having base classes doing so, see chapter ref(INHERITANCE))
may also benefit from overloaded members expecting rvalue references. The
class benefits from move operations when one or more of the composed data
members themselves support move operations.
Move operations cannot be implemented if the class type of a composed data
member does not support moving or copying. Currently, tt(stream) classes fall
into this category.
An example of a move-aware class is the class tt(std:string). A class
tt(Person) could use composition by defining tt(std::string d_name) and
tt(std::string d_address). Its move constructor would then have the following
prototype:
verb(
Person(Person &&tmp);
)
However, the following implementation of this move constructor is
incorrect:
verb(
Person::Person(Person &&tmp)
:
d_name(tmp.d_name),
d_address(tmp.d_address)
{}
)
It is incorrect as it tt(string)'s copy constructors rather than
tt(string)'s move constructors are called. If you're wondering why this
happens then remember that move operations are em(only) performed for
anonymous objects. To the compiler anything having a name isn't anonymous. And
so, by implication, having available a rvalue reference does em(not) mean that
we're referring to an anonymous object. But we em(know) that the move
constructor is only called for anonymous arguments. To use the corresponding
tt(string) move operations we have to inform the compiler that we're talking
about anonymous data members as well. For this a cast could be used (e.g.,
tt(static_cast<Person &&>(tmp))), but the C++-0x standard provides the
function
ti(std::move) to i(anonymize) a named object. The correct implementation of
tt(Person)'s move construction is, therefore:
verb(
Person::Person(Person &&tmp)
:
d_name( std::move(tmp.d_name) ),
d_address( std::move(tmp.d_address) )
{}
)
The function tt(std::move)hi(move) is (indirectly) declared by many header
files. If no header is already declaring tt(std::move) then include
ti(utility).
When a class using composition not only contains class type data members
but also other types of data (pointers, references, primitive data types),
then these other data types can be initialized as usual. Primitive data type
members can simply be copied; references can be initialized as usual en
pointers may use move operations as discussed in the previous section.
The compiler never calls move operations for variables having names. Let's
consider the implications of this by looking at the next example, assuming
tt(Class) offers a move constructor and a copy constructor:
verb(
Class factory();
void fun(Class const &other); // a
void fun(Class &&tmp); // b
void callee(Class &&tmp);
{
fun(tmp); // 1
}
int main()
{
callee(factory());
}
)
itemization(
it() At 1 function a is called. At first sight this might be surprising,
but tt(fun)'s argument is not an em(anonymous) temporary object but a
em(named) temporary object.
)
Realizing that tt(fun(tmp)) might be called twice the compiler's choice is
understandable. If tt(tmp)'s data would have been grabbed at the first call,
the second call would receive tt(tmp) without any data. But at the last call
we might know that tt(tmp) is never used again and so we might like to ensure
that tt(fun(Class &&)) is called. For this, once again, tt(std::move) is used:
verb(
fun(std::move(tmp)); // last call!
)
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