C++ - passing references to std::shared_ptr or boost::shared_ptr

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:

1. Examine the source of your entire program very carefully until you are sure the object won't die during the function body.

2. 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.

I would advise against this practice unless you and the other programmers you work with really, really know what you are all doing.

First, you have no idea how the interface to your class might evolve and you want to prevent other programmers from doing bad things. Passing a shared_ptr by reference isn't something a programmer should expect to see, because it isn't idiomatic, and that makes it easy to use it incorrectly. Program defensively: make the interface hard to use incorrectly. Passing by reference is just going to invite problems later on.

Second, don't optimize until you know this particular class is going to be a problem. Profile first, and then if your program really needs the boost given by passing by reference, then maybe. Otherwise, don't sweat the small stuff (i.e. the extra N instructions it takes to pass by value) instead worry about design, data structures, algorithms, and long-term maintainability.

Yes, taking a reference is fine there. You don't intend to give the method shared ownership; it only wants to work with it. You could take a reference for the first case too, since you copy it anyway. But for first case, it takes ownership. There is this trick to still copy it only once:

void ClassA::take_copy_of_sp(boost::shared_ptr<foo> sp) {
    m_sp_member.swap(sp);
}

You should also copy when you return it (i.e not return a reference). Because your class doesn't know what the client is doing with it (it could store a pointer to it and then big bang happens). If it later turns out it's a bottleneck (first profile!), then you can still return a reference.

Edit: Of course, as others point out, this only is true if you know your code and know that you don't reset the passed shared pointer in some way. If in doubt, just pass by value.

It is sensible to pass shared_ptrs by const&. It will not likely cause trouble (except in the unlikely case that the referenced shared_ptr is deleted during the function call, as detailed by Earwicker) and it will likely be faster if you pass a lot of these around. Remember; the default boost::shared_ptr is thread safe, so copying it includes a thread safe increment.

Try to use const& rather than just &, because temporary objects may not be passed by non-const reference. (Even though a language extension in MSVC allows you to do it anyway)

In the second case, doing this is simpler:

Class::only_work_with_sp(foo &sp)
{    
    ...  
    sp.do_something();  
    ...  
}

You can call it as

only_work_with_sp(*sp);

I would avoid a "plain" reference unless the function explicitely may modify the pointer.

A const & may be a sensible micro-optimization when calling small functions - e.g. to enable further optimizations, like inlining away some conditions. Also, the increment/decrement - since it's thread safe - is a synchronization point. I would not expect this to make a big difference in most scenarios, though.

Generally, you should use the simpler style unless you have reason not to. Then, either use the const & consistently, or add a comment as to why if you use it just in a few places.

I'll assume that you are familiar with premature optimization and are asking this either for academic purposes or because you have isolated some pre-existing code that is under-performing.

Passing by reference is okay

Passing by const reference is better, and can usually be used, as it does not force const-ness on the object pointed to.

You are not at risk of losing the pointer due to using a reference. That reference is evidence that you have a copy of the smart pointer earlier in the stack and only one thread owns a call stack, so that pre-existing copy isn't going away.

Using references is often more efficient for the reasons you mention, but not guaranteed. Remember that dereferencing an object can take work too. Your ideal reference-usage scenario would be if your coding style involves many small functions, where the pointer would get passed from function to function to function before being used.

You should always avoid storing your smart pointer as a reference. Your Class::take_copy_of_sp(&sp) example shows correct usage for that.

Assuming we are not concerned with const correctness (or more, you mean to allow the functions to be able to modify or share ownership of the data being passed in), passing a boost::shared_ptr by value is safer than passing it by reference as we allow the original boost::shared_ptr to control it's own lifetime. Consider the results of the following code...

void FooTakesReference( boost::shared_ptr< int > & ptr )
{
    ptr.reset(); // We reset, and so does sharedA, memory is deleted.
}

void FooTakesValue( boost::shared_ptr< int > ptr )
{
    ptr.reset(); // Our temporary is reset, however sharedB hasn't.
}

void main()
{
    boost::shared_ptr< int > sharedA( new int( 13 ) );
    boost::shared_ptr< int > sharedB( new int( 14 ) );

    FooTakesReference( sharedA );

    FooTakesValue( sharedB );
}

From the example above we see that passing sharedA by reference allows FooTakesReference to reset the original pointer, which reduces it's use count to 0, destroying it's data. FooTakesValue, however, can't reset the original pointer, guaranteeing sharedB's data is still usable. When another developer inevitably comes along and attempts to piggyback on sharedA's fragile existence, chaos ensues. The lucky sharedB developer, however, goes home early as all is right in his world.

The code safety, in this case, far outweighs any speed improvement copying creates. At the same time, the boost::shared_ptr is meant to improve code safety. It will be far easier to go from a copy to a reference, if something requires this kind of niche optimization.

In addition to what litb said, I'd like to point out that it's probably to pass by const reference in the second example, that way you are sure you don't accidentally modify it.