Question

Many times I needed a set of pointers. Every time that happens, I end up writing a less<> implementation for a pointer type – cast two pointers to size_t and compare the results.

My question is – is that available in the standard? I could not find anything like that. Seems like common enough case…

Update: it seems that the upcoming standard fixes all of the problems with less<> provided for pointer types and unordered_set included, too. In a few years this question will be moot.

In the mean time, the current standard has no “legal” solution to this, but size_t cast works.

Update for update: well, I’ll be gobsmacked! Not only

std::map<void *, int, std::less<void*> > myMap;

works, but even

std::map<void *, int > myMap;

as well.

And that’s in gcc 3.4.1 . I’ve been doing all the these casts for nothing, and litb is perfectly right. Even the section number he cites is exactly the same in the current standard. Hurray!

Answer 1

Two pointers can be compared with using the comparison function objects less, greater etc. Otherwise, using blanket operator< etc, this is only possible if the pointers point to elements of the same array object or one past the end. Otherwise, results are unspecified.

20.3.3/8 in C++03

For templates greater, less, greater_equal, and less_equal, the specializations for any
pointer type yield a total order, even if the built-in operators <, >, <=, >= do not.

No need to explicitly specialize and manually casting to size_t: That would lower the portability even, since the mapping of reinterpret_cast from pointers to integers is implementation defined and is not required to yield any order.

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Edit: For a more detailed answer, see this one.