Priority Queue With a Custom Comparator No Matching Constructor - c++

Question

I am trying to create a priority queue with a custom comparator but the following code gives me a compile error:
auto comparator = [](std::pair<std::vector<int>, File&> const &a, std::pair<std::vector<int>, File&> const &b) {
return a.first.front() > b.first.front();
};
std::priority_queue<std::pair<std::vector<uint64_t>, File&>,
std::vector<std::pair<std::vector<uint64_t>, File&>>,
decltype(comparator)> pq;
This is the error I am getting:
In template: no matching constructor for initialization of 'std::priority_queue<std::pair<std::vector, moderndbs::File &>, std::vector<std::pair<std::vector, moderndbs::File &>>, (lambda at

Answer 1

There's a mismatch in your question, between uint64_t and int. Assuming that this is squared away:
You need at least C++20 in order to compile the shown code. Prior to C++20, lambdas are not default-constructible, and you attempting to default-construct your std::priority_queue.
You need to explicitly pass the comparator to the constructor. Tested with gcc 10, with -std=c++17:
#include <queue>
#include <functional>
#include <vector>
#include <cstdint>
class File{};
void foo()
{
auto comparator = [](std::pair<std::vector<int>,
File &> const &a,
std::pair<std::vector<int>,
File &> const &b) {
return a.first.front() > b.first.front();
};
std::priority_queue<std::pair<std::vector<int>,
File &>,
std::vector<std::pair<std::vector<int>,
File &>
>,
decltype(comparator)> pq{comparator};
}
This fails with with a default-constructor pq. gcc 10 compiles the default-constructed pq with -std=c++20.
P.S.: consider replacing the File & with std::reference_wrapper, unless you really intend to do what the File & in std::pair will really do.