C++ priority_queue with lambda comparator error

Question

I have the following erroneous code which I am trying to compile in VC2010, but I'm getting the error C2974 this only occurs when I include the lambda expression, so I'm guessing it has something to do with that.

typedef pair<pair<int, int>, int> adjlist_edge;
priority_queue< adjlist_edge , vector<adjlist_edge>,
    [](adjlist_edge a, adjlist_edge b) -> bool {
        if(a.second > b.second){ return true; } else { return false; }
    }> adjlist_pq;

I know the form of the template definition is correct as

priority_queue<int , vector<int>, greater<int>> pq;

Works as expected. Any ideas what I'm doing wrong? Is there something obviously wrong with the lambda that looks wrong that I might be overlooking? Thanks for reading!

Answer 1

First define the lambda object, then pass it to the template's type using decltype and also pass it directly to the constructor.

auto comp = []( adjist a, adjlist b ) { return a.second > b.second; };
priority_queue< adjlist_edge , vector<adjlist_edge>, decltype( comp ) >
     adjlist_pq( comp );

Answer 2

priority_queue takes the comparator as a template argument. Lambda functions are objects, and thus can't be used as template arguments (only very few types can be, among them integral types).

You can try using decltype there:

priority_queue< adjlist_edge , vector<adjlist_edge>,
               decltype( [](adjlist_edge a, adjlist_edge b) -> bool {
                if(a.second > b.second){ return true; } else { return false; }
               })>
adjlist_pq( [](adjlist_edge a, adjlist_edge b) -> bool {
                if(a.second > b.second){ return true; } else { return false; }
             } );

Failing that (and it will), you can use function<>:

priority_queue< adjlist_edge , vector<adjlist_edge>,
                function<bool(adjlist_edge,adjlist_edge)> >
adjlist_pq( [](adjlist_edge a, adjlist_edge b) -> bool {
                if(a.second > b.second){ return true; } else { return false; }
            } );