C++ std -- queue

priority queue

Constructor

Priority queue has signature std::priority_queue<T,Container=vector,Compare=std::less> , so you can see that comparator class is provided as a template parameter. Below code does not compile

priority_queue<int> pq(std::greater<int>{});

with error message note: candidate constructor template not viable:.... So how should we do it?

auto comp = std::greater<int>();
priority_queue<int, std::vector<int>, decltype(comp)> pq(comp);

Ah! Nice decltype. Note that, we cannot omit the second template parameter which is quite annoying.

Another note about above code is std::greater<int>{}. Why uses curly braces instead of parenthesis. Checkout Most vexing parsing. Basically, C/C++ will interpret it as

priority_queue<int, std::vector<int>, decltype(comp)> pq(std::greater<int>);

i.e., a function definition pq with an anomalous parameter. C/C++ allow superficial parenthesis around function parameter.