boost::bind seems more like evidence in the case against C++ than an advertisement for boost.
Well, yes. That's sort of the point. You can write old-style C++, or your can write modern C++, but the library designers are trying to make you think carefully when you're doing both, because that is a source of memory lifecycle bugs. Using the boost (now standard) pointers universally tends to dramatically reduce the chances of memory-related bugs in C++ code.
Not using shared_ptr is a code smell. Sometimes you have to do it, but if you're avoiding the use of the smart pointer classes in blue sky code, you're doing something wrong.
If some external code is passing you pointers, then using shared_ptr is exactly the wrong thing to do, because you (presumably) don't own the memory. And if you do own the memory, there's no problem putting it in a shared_ptr. The fact that you can't blindly stuff everything into a shared_ptr is a feature, not a bug.
class A { ... };
class B : public A { ... };
// Totally works
class AFactory {
public:
virtual A* makeOne();
}
class BFactory : public AFactory {
public:
virtual B* makeOne();
}
// totally doesn't
class SharedAFactory {
public:
virtual boost::shared_ptr<A> makeOne();
};
class SharedBFactory {
public:
virtual boost::shared_ptr<B> makeOne();
};
If not, the only problem here is that C++ won't let you have a virtual function override that only differs by return type. And since it can't tell that shared_ptr<A> and shared_ptr<B> are related by value_type, it complains (this is the larger point that I granted you earlier). But the type theatrics are hiding a bigger problem....
In this example, you can either fix the type error by making makeOne() return a boost::shared_ptr<A> in all factories (in which case, you really do want a polymorphic interface), or you can make a new function that returns a different pointer type (in which case, you don't). The type system here is doing you a favor, and telling you that your design is bogus -- you shouldn't be creating a polymorphic factory interface that returns different types (because that isn't actually polymorphic, is it?)
This isn't totally academic: I find that in nearly all type-error situations like this one, the real problem can be traced back to sloppy coding.
"locking the memory bus to do atomic operations on a shared_ptr totally blows when you're expected to turn a request around in a few microseconds."
There's always a cost to reference counting, and therefore, it's not always appropriate. That said, the only time you should be triggering atomic operations are on pointer copies and allocations. If you're doing allocations per request, kiss your microsecond response time goodbye -- the reference count overhead is the least of your worries. And you can avoid unnecessary pointer copies by passing shared_ptr<> objects by reference. With a small amount of care, it's possible to reduce shared_ptr<> overhead to essentially zero.
Covariant return types are a powerful feature in a language that doesn't support other forms of return type polymorphism. shared_ptrs function so similarly to pointers that it can be surprising when they don't support this behavior. Surprise and capability mismatch with pointers are strikes against shared_ptr. But not sufficient ones to stop me from using them.
No, they're not. One is returning a pointer to A, and the other is returning a pointer to B (which just happens to be a subclass of A). It matters, and just because you can do it with a naked pointer doesn't mean that you should. The problem you've described is a code smell.
If I hand you a pointer to B, you (client code) can do everything that is exposed by B. If I hand you a pointer to A, you can only do the set of things exposed by A. And that's a fundamentally different interface guarantee.
I have a polymorphic type that should be copyable. Because of the limitations of C++ copy constructors and operators, my only option is to expose a virtual method (call it "copy"). The non-smelly semantics we want are such that if you call "copy" on an object, you get an identical copy of the same type. So if you call "copy" on a pointer of type A, you get an A. Call it on a B, you get a B.
The contract itself is polymorphic. It doesn't commit to returning any particular flavor of A.
We can implement this method if we return naked pointers. But if we want to be safe and return a smart pointer, we have to introduce dynamic casting or other worse smells.
Well, yeah...because those semantics are just as smelly as the one you described before. What you describe isn't a copy constructor at all, and trying to make a copy constructor do what you want is a dangerous thing. A "constructor" that takes any subclass of class A and returns an instance of that subclass is inherently brittle: add a new subclass of A, and you've got to update the "constructor" to know about the new type. On top of that, your "constructor" has to introspect into the type to determine what to initialize when, and that's slow.
There are easy ways to get the behavior your want (e.g. make a template function that calls the appropriate class' copy constructor, or -- closest to what you want -- make a template copy constructor function on the class itself), but complaining that you can't make a bog-standard copy constructor do polymorphic construction kind of misses the point.
Here's a better discussion of the problem than I have the space to go into here:
But just to clarify: no base class implementation is aware of derived classes in the copying case, so your smell doesn't exist. See Scott Meyers' "virtual copy constructor" snippet half way down the page: http://books.google.com/books?id=azvE8V0c-mYC&lpg=PT159&...
You can't do it that neatly without covariant return types. Ergo, you can't do that while returning safe, smart pointers.
Not using shared_ptr a code smell? If you're not able to keep track of your objects yourself, you should not be using C or C++. It's not that hard like you would like us believe.
Also, apart from the cost of maintaining reference counts, shared_ptr makes an extra allocation for the control block. Unless you take care and use make_shared, you end up with allocations that are twice as slow.
On a cynical side: Boost has become Dave Abrahams' business (1) He (and co.) have a vested interest in making you believe that the Boost-way is "the" way of writing "modern" (as defined by HIM and his followers) C++ code. I just wonder whether he had that much foresight, or just took the chance when he saw the opportunity.
(1) I remember once asking about obtaining slides about fusion from a past BoostCon conference and got a blank "no". Slides were available only to the people who paid to attend the conference.
Yes. It's part of the C++ standard. All rants against Dave Abrahams and Boost aside, not using the standard library to do standard things is a code smell.