std::priority_queue: Custom ordering without defining comparator class

Question

I want to have a priority queue with custom ordering, but lazy as I am, I don't want to define a comparator class implementing operator().

I really would like something like this to compile:

std::priority_queue<int, std::vector<int>, 
    boost::bind(some_function, _1, _2, obj1, obj2)> queue;

where some_function is a bool returning function taking four arguments, the first and second being ints of the queue, and the two last ones some objects needed for calculating the ordering (const references).

(error: ‘boost::bind’ cannot appear in a constant-expression)

But this doesn't compile. Even a more simple

std::priority_queue<int, std::vector<int>, &compare> queue;

won't compile, with compare being a binary function returning bool.

(error: type/value mismatch at argument 3 in template parameter list for ‘template class std::priority_queue’; expected a type, got ‘compare’)

Any suggestions?

You have no closing paren on the boost::bind here - before the > on the queue template params. Is that a typo in posted code or in what you tried to compile? — Steve Townsend
– Steve Townsend, Commented Nov 15, 2010 at 16:48

Steve M · Accepted Answer · 2010-11-15 16:51:02Z

11

This could work:

std::priority_queue<int, std::vector<int>, 
    boost::function<bool(int,int)> >

Then pass in your bind expression to the queue's constructor.

P.S. you're getting those compilation errors because you're putting runtime-evaluated expressions where a typename or constant expression are expected.

edited Nov 15, 2010 at 16:51

answered Nov 15, 2010 at 16:46

Steve M

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1 Comment

Charles Salvia Over a year ago

+1, This is a good solution. I'll just note that unfortunately, this isn't exactly the same thing as statically supplying a specific template function/functor parameter, because boost::function uses dynamic allocation to create variable function objects. So, you won't get the same kind of compiler-generated inlined efficiency that you would get with a custom, statically supplied function parameter.

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