The point of a distinct shared_ptr instance is to guarantee (as far as possible) that as long as this shared_ptr is in scope, the object it points to will still exist, because its reference count will be at least 1.
Class::only_work_with_sp(boost::shared_ptr<foo> sp)
{
// sp points to an object that cannot be destroyed during this function
}
So by using a reference to a shared_ptr, you disable that guarantee. So in your second case:
Class::only_work_with_sp(boost::shared_ptr<foo> &sp) //Again, no copy here
{
...
sp->do_something();
...
}
How do you know that sp->do_something() will not blow up due to a null pointer?
It all depends what is in those '...' sections of the code. What if you call something during the first '...' that has the side-effect (somewhere in another part of the code) of clearing a shared_ptr to that same object? And what if it happens to be the only remaining distinct shared_ptr to that object? Bye bye object, just where you're about to try and use it.
So there are two ways to answer that question:
Examine the source of your entire program very carefully until you are sure the object won't die during the function body.
Change the parameter back to be a distinct object instead of a reference.
General bit of advice that applies here: don't bother making risky changes to your code for the sake of performance until you've timed your product in a realistic situation in a profiler and conclusively measured that the change you want to make will make a significant difference to performance.
Update for commenter JQ
Here's a contrived example. It's deliberately simple, so the mistake will be obvious. In real examples, the mistake is not so obvious because it is hidden in layers of real detail.
We have a function that will send a message somewhere. It may be a large message so rather than using a std::string that likely gets copied as it is passed around to multiple places, we use a shared_ptr to a string:
void send_message(std::shared_ptr<std::string> msg)
{
std::cout << (*msg.get()) << std::endl;
}
(We just "send" it to the console for this example).
Now we want to add a facility to remember the previous message. We want the following behaviour: a variable must exist that contains the most recently sent message, but while a message is currently being sent then there must be no previous message (the variable should be reset before sending). So we declare the new variable:
std::shared_ptr<std::string> previous_message;
Then we amend our function according to the rules we specified:
void send_message(std::shared_ptr<std::string> msg)
{
previous_message = 0;
std::cout << *msg << std::endl;
previous_message = msg;
}
So, before we start sending we discard the current previous message, and then after the send is complete we can store the new previous message. All good. Here's some test code:
send_message(std::shared_ptr<std::string>(new std::string("Hi")));
send_message(previous_message);
And as expected, this prints Hi! twice.
Now along comes Mr Maintainer, who looks at the code and thinks: Hey, that parameter to send_message is a shared_ptr:
void send_message(std::shared_ptr<std::string> msg)
Obviously that can be changed to:
void send_message(const std::shared_ptr<std::string> &msg)
Think of the performance enhancement this will bring! (Never mind that we're about to send a typically large message over some channel, so the performance enhancement will be so small as to be unmeasureable).
But the real problem is that now the test code will exhibit undefined behaviour (in Visual C++ 2010 debug builds, it crashes).
Mr Maintainer is surprised by this, but adds a defensive check to send_message in an attempt to stop the problem happening:
void send_message(const std::shared_ptr<std::string> &msg)
{
if (msg == 0)
return;
But of course it still goes ahead and crashes, because msg is never null when send_message is called.
As I say, with all the code so close together in a trivial example, it's easy to find the mistake. But in real programs, with more complex relationships between mutable objects that hold pointers to each other, it is easy to make the mistake, and hard to construct the necessary test cases to detect the mistake.
The easy solution, where you want a function to be able to rely on a shared_ptr continuing to be non-null throughout, is for the function to allocate its own true shared_ptr, rather than relying on a reference to an existing shared_ptr.
The downside is that copied a shared_ptr is not free: even "lock-free" implementations have to use an interlocked operation to honour threading guarantees. So there may be situations where a program can be significantly sped up by changing a shared_ptr into a shared_ptr &. But it this is not a change that can be safely made to all programs. It changes the logical meaning of the program.
Note that a similar bug would occur if we used std::string throughout instead of std::shared_ptr<std::string>, and instead of:
previous_message = 0;
to clear the message, we said:
previous_message.clear();
Then the symptom would be the accidental sending of an empty message, instead of undefined behaviour. The cost of an extra copy of a very large string may be a lot more significant than the cost of copying a shared_ptr, so the trade-off may be different.
