Can you use a boost::shared_ptr as a key for a map?

Question

I may need to rethink my overall design a bit more, but as it stands, it looks like I may want to do something like:

class A; 
class B;
std::map<boost::shared_ptr<const A>, B> APtrToBMap;

I've tried this, and it does seem to work in a simple case (the compiler didn't complain, and simple tests seem to work). But I'm having second thoughts about this approach. I suspect there are some gotchas in there that I'm not aware of.

So, is the above valid in a practical sense? Or is there some flaw I'm not aware of when I do this?

@Sam, just because you haven't discovered a problem yet doesn't mean it's nonexistant. I think it's a great idea to ask about potential problems for a non-standard usage like this. — Mark Ransom, Nov 10 '10 at 17:02
Thank you everyone for your responses. As some of you noted, it is mainly a symptom of a bad design; I was trying to solve a problem in the wrong place. I've since reexamined my design and won't need to do the bad map I questioned about. The responses as to why this is a bad idea too were really enlightening; I knew in my gut it wouldn't quite do what I was looking to do; I just couldn't see why until now. (In fact what I was trying to do wouldn't work with the map I templated above) — Taeolas, Nov 10 '10 at 18:55

score 24 · Accepted Answer · answered Nov 10 '10 at 15:27

24

The reference has this to say:

template<class T, class U>
  bool operator<(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws

Returns: an unspecified value such that

operator< is a strict weak ordering as described in section 25.3 [lib.alg.sorting] of the C++ standard;
under the equivalence relation defined by operator<, !(a < b) && !(b < a), two shared_ptr instances are equivalent if and only if they share ownership or are both empty.

Throws: nothing.

Notes: Allows shared_ptr objects to be used as keys in associative containers.

answered Nov 10 '10 at 15:27

UncleBens

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I'm glad you found this. I came to the opposite conclusion based on a quick peek at the header, but that was based on my own flawed analysis. – Mark Ransom Nov 10 '10 at 15:40
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but why one would want to use shared_ptr as a key in a container? – davka Nov 10 '10 at 16:13
Thank you for the answer and the clarification. As I suspected I need to reexamine my approach to this. In my situation, I was looking for an easy way to track shared_ptr and "B", but this wouldn't work. – Taeolas Nov 10 '10 at 16:24
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@davka: so that you can put shared_ptr into a `set`. – rwong May 22 '12 at 18:00

score 4 · Answer 2 · answered Nov 10 '10 at 16:40

4

Yes, you can use a shared pointer as the key for the std::map, but that is a sure sign of broken design. It would be ordered according to some random memory locations.

Your question is a bit vague. If you say how would you use that map and what are you trying to archive, you might get more useful answers.

answered Nov 10 '10 at 16:40

BЈовић

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A map with random ordering is still useful for quick lookup of data that is associated with but not part of the object. I wouldn't jump to the conclusion of a broken design. – Mark Ransom Nov 10 '10 at 16:58
@Mark Can you give an example? If the map's key is a pointer, then why the map's value is not part of the object? The effect is the same. – BЈовић Nov 10 '10 at 17:03
What if you want to search for data based on a particular object? I've used this quite a few times in attaching data to items in UI lists and such when the API didn't provide a reasonable method of doing so. – Edward Strange Nov 10 '10 at 17:18
@Noah Then write a wrapper. By doing so, you can also ease the unit testing of your code, because you can replace your objects with mock objects. – BЈовић Nov 10 '10 at 19:36

score 0 · Answer 3 · answered Nov 28 '12 at 08:42

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Careful if you're using this in conjunction with boost::python! All operations in C++ based on comparison of shared_ptr objects may fail if you pass in those shared_ptr objects from Python

answered Nov 28 '12 at 08:42

Fabian Hachenberg

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score 0 · Answer 4 · answered Nov 10 '10 at 16:24

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I'm not 100% familiar with shared pointer, but if there is a way to get access to the original pointer, you can use that as the key (if you are still worried for some reason).

answered Nov 10 '10 at 16:24

robev

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But then the map wouldn't keep the shared_ptr from destroying itself. If you were to put the shared_ptr itself into the map, then you could make it such that the only reference to the ptr was found within the map. – Bill Lynch Nov 10 '10 at 16:52
I don't understand...he just wants a map of the pointer to the B object, so I said if he's afraid of using the shared ptr for some reason he can use the pointer itself. – robev Nov 10 '10 at 16:57
Or do you mean that having a copy of the shared_ptr in the map, it'll keep the object from being deleted? – robev Nov 10 '10 at 16:57
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You can do this without having to STORE the raw pointer. You can store the shared pointer and override the comparison operator of the map. – Edward Strange Nov 10 '10 at 17:19

Can you use a boost::shared_ptr as a key for a map?

4 Answers4