Re: Pseudo "const references" and smart pointers
On 2011-06-17 00:58, kelvSYC wrote:
I'm not sure how to design this "the C++ way" (and I'm getting weird
runtime errors relating to object destruction), so I'll ask here:
Let me start with the remark, that a short programmatic description
would have simplified your explanations a lot. I try to fill the holes
iteratively.
Suppose I have a class called Function which currently stores an
instance of a class Group.
I assume you mean the following kind of model code ignoring aspects of
access:
struct Group {};
struct Function {
Group g;
Function(Group g) : g(g) {}
virtual ~Function(){}
virtual void evaluate() = 0;
};
This instance is not modified in any way
throughout the lifetime of the object. This class has a polymorphic
function evaluate(), where the Group instance is used (evaluate() is
pure virtual, and Function itself is an incomplete type). Function is
not default-constructible (a function MUST be tied to a Group), but
must be assignable and copy-constructible as it must be put into a
list.
Does that mean that Group is also assignable and copy-constructible?
What shall happen during a copy/assign of Function with the Group
sub-object? Cloning? Flat copy of the address?
In the interest of saving space (say, if Group takes up a lot of
space), then, I'd change have replace the copy of Group with something
like a const Group&, but retaining the ability to assign.
OK, this gives me:
struct Group {};
struct Function {
const Group* g;
Function(const Group& g) : g(&g) {}
virtual ~Function(){}
virtual void evaluate() = 0;
};
But we have to keep in mind that this approach means that copies of
Function are not completely independent, because they share the same
Group object.
Being
paranoid about raw pointer members (though the input Group is always
stack-allocated in my application, there might be the case it won't be
in the future), I had been thinking about using some kind of smart
pointer object. Is there something that I can use in the C++ standard
library or boost (which my project uses) for this?
A smart pointer makes sense. If you need to write against C++03
compatible compiler and library I suggest to use boost::shared_ptr,
because you can assign a custom deleter. This can also be a noop deleter
which works fine for your stack-allocated objects of type Group.
If you have a C++0x compatible compiler and library, std::shared_ptr
(which is more or less equivalent to boost::shared_ptr) seems suitable.
Usually I would also recommend std::unique_ptr in this case, but this
type is move-only, thus Function would be move-only as well and you need
to move around these objects. The advantage is, that you don't create
multiple copies that work on the same Group object. Whether this
approach works for you, depends on the use cases for Function.
std::unique_ptr also allows the provision of a custom deleter, but the
difference compared to std::/boost::shared_ptr is, that the deleter is
part of the unique_ptr type (as second template parameter).
As an aside, if alternately a const reference is probably for the
best, is there a standard exception that can be thrown on an
incompatible assignment (in the case that a Function object is being
assigned to one where the associated Group is different)?
I would not say that the reference is the "best" (my first assumption
was that you wanted a pointer) and I'm missing the criteria for a proper
measurement. In regard to your question of a standard exception for
incompatible assignment, there is nothing special coming into my mind.
HTH & Greetings from Bremen,
Daniel Kr?gler
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