I recently came across some C++ code that looked like this:
class SomeObject
{
private:
// NOT a pointer
BigObject foobar;
public:
BigObject * getFoobar() const
{
return &foobar;
}
};
I asked the programmer why he didn't just make foobar a pointer, and he said that this way he didn't have to worry about allocating/deallocating memory. I asked if he considered using some smart pointer, he said this worked just as well.
Is this bad practice? It seems very hackish.
That's perfectly reasonable, and not "hackish" in any way; although it might be considered better to return a reference to indicate that the object definitely exists. A pointer might be null, and might lead some to think that they should delete it after use.
The object has to exist somewhere, and existing as a member of an object is usually as good as existing anywhere else. Adding an extra level of indirection by dynamically allocating it separately from the object that owns it makes the code less efficient, and adds the burden of making sure it's correctly deallocated.
Of course, the member function can't be const if it returns a non-const reference or pointer to a member. That's another advantage of making it a member: a const qualifier on SomeObject applies to its members too, but doesn't apply to any objects it merely has a pointer to.
The only danger is that the object might be destroyed while someone still has a pointer or reference to it; but that danger is still present however you manage it. Smart pointers can help here, if the object lifetimes are too complex to manage otherwise.
You are returning a pointer to a member variable not a reference. This is bad design.
Your class manages the lifetime of foobar object and by returning a pointer to its members you enable the consumers of your class to keep using the pointer beyond the lifetime of SomeObject object. And also it enables the users to change the state of SomeObject object as they wish.
Instead you should refactor your class to include the operations that would be done on the foobar in SomeObject class as methods.
ps. Consider naming your classes properly. When you define it is a class. When you instantiate, then you have an object of that class.
It's generally considered less than ideal to return pointers to internal data at all; it prevents the class from managing access to its own data. But if you want to do that anyway I see no great problem here; it simplifies the management of memory.
Is this bad practice? It seems very hackish.
It is. If the class goes out of scope before the pointer does, the member variable will no longer exist, yet a pointer to it still exists. Any attempt to dereference that pointer post class destruction will result in undefined behaviour - this could result in a crash, or it could result in hard to find bugs where arbitrary memory is read and treated as a BigObject.
if he considered using some smart pointer
Using smart pointers, specifically std::shared_ptr<T> or the boost version, would technically work here and avoid the potential crash (if you allocate via the shared pointer constructor) - however, it also confuses who owns that pointer - the class, or the caller? Furthermore, I'm not sure you can just add a pointer to an object to a smart pointer.
Both of these two points deal with the technical issue of getting a pointer out of a class, but the real question should be "why?" as in "why are you returning a pointer from a class?" There are cases where this is the only way, but more often than not you don't need to return a pointer. For example, suppose that variable needs to be passed to a C API which takes a pointer to that type. In this case, you would probably be better encapsulating that C call in the class.
As long as the caller knows that the pointer returned from getFoobar() becomes invalid when the SomeObject object destructs, it's fine. Such provisos and caveats are common in older C++ programs and frameworks.
Even current libraries have to do this for historical reasons. e.g. std::string::c_str, which returns a pointer to an internal buffer in the string, which becomes unusable when the string destructs.
Of course, that is difficult to ensure in a large or complex program. In modern C++ the preferred approach is to give everything simple "value semantics" as far as possible, so that every object's life time is controlled by the code that uses it in a trivial way. So there are no naked pointers, no explicit new or delete calls scattered around your code, etc., and so no need to require programmers to manually ensure they are following the rules.
(And then you can resort to smart pointers in cases where you are totally unable to avoid shared responsibility for object lifetimes.)
Two unrelated issues here:
1) How would you like your instance of SomeObject to manage the instance of BigObject that it needs? If each instance of SomeObject needs its own BigObject, then a BigObject data member is totally reasonable. There are situations where you'd want to do something different, but unless that situation arises stick with the simple solution.
2) Do you want to give users of SomeObject direct access to its BigObject? By default the answer here would be "no", on the basis of good encapsulation. But if you do want to, then that doesn't change the assessment of (1). Also if you do want to, you don't necessarily need to do so via a pointer -- it could be via a reference or even a public data member.
A third possible issue might arise that does change the assessment of (1):
3) Do you want to give users of SomeObject direct access to an instance of BigObject that they continue using beyond the lifetime of the instance of SomeObject that they got it from? If so then of course a data member is no good. The proper solution might be shared_ptr, or for SomeObject::getFooBar to be a factory that returns a different BigObject each time it's called.
In summary:
Other than the fact it doesn't compile (getFooBar() needs to return const BigObject*), there is no reason so far to suppose that this code is wrong. Other issues could arise that make it wrong.
It might be better style to return const & rather than const *. Which you return has no bearing on whether foobar should be a BigObject data member.
There is certainly no "just" about making foobar a pointer or a smart pointer -- either one would necessitate extra code to create an instance of BigObject to point to.
A pointer that is passed-in-by-reference. Why? aren't pointers just references anyways? What's really happening to this parameter?
void someFunc(MyPtr*& Object)
{
}
Simply speaking, it gives you the ability to change the pointer itself: it can be changed to point to another location in the function.
And the change will be reflected outside.
It enable you to:
void someFunc(MyPtr*& Object)
{
//Modify what Object is pointing to
Object=&old_Object;
//You can also allocate memory, depending on your requirements
Object=new MyPtr;
//Modify the variable Object points to
*Object=another_object;
}
Other's will have to vote to verify this cause I'm a bit rusty on my C++ but I believe the idea here is you'd pass in a pointer by reference, that is instead of creating a new space to store the pointer itself you use a reference to the pointer so if you were to modify the pointer not just the value it would be modified after returning from the function, whereas otherwise all you could do is modify the value at position passed in. Hope that makes sense.
The difference to passing just a pointer is that if the pointer is changed (Object = x) then this change will be seen by the calling function. You could achieve the same when you pass MyPtr** Object and dereference the pointer *Object = x;. With the second approach you could pass NULL to the function. This is not possible for references.
You are not quite right. The pointer content is passed by reference but the pointer itself is still passed by value, i.e. reassinging it to some other pointer will not be reflected upon the exit from the method because the pointer will be set to point to the same memory block as before the call. Think of it as a simple int variable. However with &* or ** you can reassign the pointer and that will be visible outside the scope of this method.
Why?
For the same reason that you would pass in anything else by reference.
aren't pointers just references anyways?
Dear god, no. Not even remotely the same thing. Look, you can try to build a mental model of a reference by starting with a pointer, but by the time you've fixed up all the differences, you have a horrible illogical mess.
References are a much simpler and more intuitive concept, and there are only "historical reasons" for trying to understand pointers before them. Modern C++ uses raw pointers only rarely, and treats them as an implementation detail as much as possible.
A reference is another name for an already-existing thing. That's it. When used as a function parameter, they thus allow the called function to refer to the caller's data.
It also means the pointer can be 0 (NULL) which can having meaning to the method. A reference must always be valid and cannot be made 'nothing'
I'm currently doing my first real project in C++ and so, fairly new to pointers. I know what they are and have read some basic usage rules. Probably not enough since I still do not really understand when to use them, and when not.
The problem is that most places just mention that most people either overuse them or underuse them. My question is, when to use them, and when not?.
Currently, in many cases i'm asking myself, should I use a pointer here or just pass the variable itself to the function.
For instance, I know that you can send a pointer to a function so the function can actually alter the variable itself instead of a copy of it. But when you just need to get some information of the object once (for instance the method needs a getValue() something), are pointers usefull in that case?
I would love to see either reactions but also links that might be helpfull. Since it is my first time using C++ I do not yet have a good C++ book (was thinking about buying one if I keep on using c++ which I probably will).
For the do's and dont's of C++:
Effective C++ and More Effective C++ by Scott Meyers.
For pointers (and references):
use pass by value if the type fits into 4 Bytes and don't want to have it changed after the return of the call.
use pass by reference to const if the type is larger and you don't want to have it changed after the return of the call.
use pass by reference if the parameter can't be NULL
use a pointer otherwise.
dont't use raw pointers if you don't need to. Most of the time, a smart pointer (see Boost) is the better option.
From the c++ faq:
Use references when you can, and
pointers when you have to.
https://isocpp.org/wiki/faq/references#refs-vs-ptrs
1) I tend to use member variables scoped with the class. They are constructed in the initializer of the class, and I don't need to worry about pointers.
2) You can pass by reference to a function, and not worry about passing pointers. This effectively will pass a pointer to the method / function that can be used as if you passed the class, but without the overhead of copying the class itself.
3) If I need to control the lifetime of an object that is independent of my main application architecture's classes... then I will use an auto_ptr from the STL to automatically handle the pointer's destruction when no one longer references it. Check it out - it's the way to go.
Use it whenever you are dealing with allocated memory or passing arguments by reference to a method; I don't think there is a rule for not using pointers.
My rules of thumb:
Always pass function parameters as const references,
unless they are built-in types, in which case they are copied (and const/non-const becomes a question of style as the caller isn't affected) or
unless they are meant to be changed inside the function so that the changes reflect at the caller's, in which case they are passed by non-const reference or
unless the function should be callable even if callers don't have an object to pass, then they are passed as pointers, so that callers can pass in NULL pointers instead (apply #1 and #3 to decide whether to pass per const T* or per T*)
Streams must always be passed around as non-const references.
Generally, when you can use references instead of pointers it is a good idea. A reference must have a target (no NULL pointer violations), they allow the same semantics as pointers when being passed as arguments to a function, and they are generally nicer to use for beginners (or those not coming from a C background).
Pointers are required when you want to do dynamic allocation of memory; when you need to deal with an unknown amount of things that will be later specified. In this case the interface to access memory is through new and delete which deal in pointers.
My philosophy is to always pass by value, unless you need to modify the variable passed or copying the object is expensive. In both these cases, consider using a reference instead of a pointer first: if you don't need to change which object you're referencing, nor do you need a possible extremal value (NULL pointer), you can use a reference.
Don't forget about iterators either.
All good answers above. Additionally, if you are performing some processor-intensive work, it's important to realize that dereferencing a pointer will likely be a cache miss on your processor. It's a good idea to keep your data accessible with minimal pointer dereferences.
Class attribute: pointer
Variables declared in methods: no pointers, so we avoid memory leaks.
In this way, prevent memory leaks and controlle attribute's consistency.
Salu2.
If I have a function that needs to work with a shared_ptr, wouldn't it be more efficient to pass it a reference to it (so to avoid copying the shared_ptr object)?
What are the possible bad side effects?
I envision two possible cases:
1) inside the function a copy is made of the argument, like in
ClassA::take_copy_of_sp(boost::shared_ptr<foo> &sp)
{
...
m_sp_member=sp; //This will copy the object, incrementing refcount
...
}
2) inside the function the argument is only used, like in
Class::only_work_with_sp(boost::shared_ptr<foo> &sp) //Again, no copy here
{
...
sp->do_something();
...
}
I can't see in both cases a good reason to pass the boost::shared_ptr<foo> by value instead of by reference. Passing by value would only "temporarily" increment the reference count due to the copying, and then decrement it when exiting the function scope.
Am I overlooking something?
Just to clarify, after reading several answers: I perfectly agree on the premature-optimization concerns, and I always try to first-profile-then-work-on-the-hotspots. My question was more from a purely technical code-point-of-view, if you know what I mean.
I found myself disagreeing with the highest-voted answer, so I went looking for expert opinons and here they are.
From http://channel9.msdn.com/Shows/Going+Deep/C-and-Beyond-2011-Scott-Andrei-and-Herb-Ask-Us-Anything
Herb Sutter: "when you pass shared_ptrs, copies are expensive"
Scott Meyers: "There's nothing special about shared_ptr when it comes to whether you pass it by value, or pass it by reference. Use exactly the same analysis you use for any other user defined type. People seem to have this perception that shared_ptr somehow solves all management problems, and that because it's small, it's necessarily inexpensive to pass by value. It has to be copied, and there is a cost associated with that... it's expensive to pass it by value, so if I can get away with it with proper semantics in my program, I'm gonna pass it by reference to const or reference instead"
Herb Sutter: "always pass them by reference to const, and very occasionally maybe because you know what you called might modify the thing you got a reference from, maybe then you might pass by value... if you copy them as parameters, oh my goodness you almost never need to bump that reference count because it's being held alive anyway, and you should be passing it by reference, so please do that"
Update: Herb has expanded on this here: http://herbsutter.com/2013/06/05/gotw-91-solution-smart-pointer-parameters/, although the moral of the story is that you shouldn't be passing shared_ptrs at all "unless you want to use or manipulate the smart pointer itself, such as to share or transfer ownership."
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.
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 would advocate passing shared pointer by const reference - a semantics that the function being passed with the pointer does NOT own the pointer, which is a clean idiom for developers.
The only pitfall is in multiple thread programs the object being pointed by the shared pointer gets destroyed in another thread. So it is safe to say using const reference of shared pointer is safe in single threaded program.
Passing shared pointer by non-const reference is sometimes dangerous - the reason is the swap and reset functions the function may invoke inside so as to destroy the object which is still considered valid after the function returns.
It is not about premature optimization, I guess - it is about avoiding unnecessary waste of CPU cycles when you are clear what you want to do and the coding idiom has firmly been adopted by your fellow developers.
Just my 2 cents :-)
It seems that all the pros and cons here can actually be generalised to ANY type passed by reference not just shared_ptr. In my opinion, you should know the semantic of passing by reference, const reference and value and use it correctly. But there is absolutely nothing inherently wrong with passing shared_ptr by reference, unless you think that all references are bad...
To go back to the example:
Class::only_work_with_sp( foo &sp ) //Again, no copy here
{
...
sp.do_something();
...
}
How do you know that sp.do_something() will not blow up due to a dangling pointer?
The truth is that, shared_ptr or not, const or not, this could happen if you have a design flaw, like directly or indirectly sharing the ownership of sp between threads, missusing an object that do delete this, you have a circular ownership or other ownership errors.
One thing that I haven't seen mentioned yet is that when you pass shared pointers by reference, you lose the implicit conversion that you get if you want to pass a derived class shared pointer through a reference to a base class shared pointer.
For example, this code will produce an error, but it will work if you change test() so that the shared pointer is not passed by reference.
#include <boost/shared_ptr.hpp>
class Base { };
class Derived: public Base { };
// ONLY instances of Base can be passed by reference. If you have a shared_ptr
// to a derived type, you have to cast it manually. If you remove the reference
// and pass the shared_ptr by value, then the cast is implicit so you don't have
// to worry about it.
void test(boost::shared_ptr<Base>& b)
{
return;
}
int main(void)
{
boost::shared_ptr<Derived> d(new Derived);
test(d);
// If you want the above call to work with references, you will have to manually cast
// pointers like this, EVERY time you call the function. Since you are creating a new
// shared pointer, you lose the benefit of passing by reference.
boost::shared_ptr<Base> b = boost::dynamic_pointer_cast<Base>(d);
test(b);
return 0;
}
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.
Sandy wrote: "It seems that all the pros and cons here can actually be generalised to ANY type passed by reference not just shared_ptr."
True to some extent, but the point of using shared_ptr is to eliminate concerns regarding object lifetimes and to let the compiler handle that for you. If you're going to pass a shared pointer by reference and allow clients of your reference-counted-object call non-const methods that might free the object data, then using a shared pointer is almost pointless.
I wrote "almost" in that previous sentence because performance can be a concern, and it 'might' be justified in rare cases, but I would also avoid this scenario myself and look for all possible other optimization solutions myself, such as to seriously look at adding another level of indirection, lazy evaluation, etc..
Code that exists past it's author, or even post it's author's memory, that requires implicit assumptions about behavior, in particular behavior about object lifetimes, requires clear, concise, readable documentation, and then many clients won't read it anyway! Simplicity almost always trumps efficiency, and there are almost always other ways to be efficient. If you really need to pass values by reference to avoid deep copying by copy constructors of your reference-counted-objects (and the equals operator), then perhaps you should consider ways to make the deep-copied data be reference counted pointers that can be copied quickly. (Of course, that's just one design scenario that might not apply to your situation).
I used to work in a project that the principle was very strong about passing smart pointers by value. When I was asked to do some performance analysis - I found that for increment and decrement of the reference counters of the smart pointers the application spends between 4-6% of the utilized processor time.
If you want to pass the smart pointers by value just to avoid having issues in weird cases as described from Daniel Earwicker make sure you understand the price you paying for it.
If you decide to go with a reference the main reason to use const reference is to make it possible to have implicit upcasting when you need to pass shared pointer to object from class that inherits the class you use in the interface.
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.
struct A {
shared_ptr<Message> msg;
shared_ptr<Message> * ptr_msg;
}
pass by value:
void set(shared_ptr<Message> msg) {
this->msg = msg; /// create a new shared_ptr, reference count will be added;
} /// out of method, new created shared_ptr will be deleted, of course, reference count also be reduced;
pass by reference:
void set(shared_ptr<Message>& msg) {
this->msg = msg; /// reference count will be added, because reference is just an alias.
}
pass by pointer:
void set(shared_ptr<Message>* msg) {
this->ptr_msg = msg; /// reference count will not be added;
}
Every code piece must carry some sense. If you pass a shared pointer by value everywhere in the application, this means "I am unsure about what's going on elsewhere, hence I favour raw safety". This is not what I call a good confidence sign to other programmers who could consult the code.
Anyway, even if a function gets a const reference and you are "unsure", you can still create a copy of the shared pointer at the head of the function, to add a strong reference to the pointer. This could also be seen as a hint about the design ("the pointer could be modified elsewhere").
So yes, IMO, the default should be "pass by const reference".