std::map std::priority_queue && lambda comparator

Question

Bonjour

I would like to use a map of strings and priority queue with a lamda based custom comparator. I wrote something similar to the code below but it does'nt compile and I believe I almost understand why (any good explainations are more than welcome)

Q: How should I modify the code below such that it compile.

auto MyComparator = [](const Person &a, const Person &b) {
    return a.YearOfBirth > b.YearOfBirth;  
};

 std::map<std::string, std::priority_queue<Person, std::vector<Person>, decltype(MyComparator)>> Familly(MyComparator);

I did some testing before to come here and the code below code compile and works as expected but I'm not sure to fully understand why.

struct MyComparator {
  bool operator()(const Person &a, const Person &b) {
    return a.YearOfBirth > b.YearOfBirth;
  }
};

std::map<std::string, std::priority_queue<Person, std::vector<Person>, MyComparator>> Familly;

Note: the code above also works if you use a class rather than a struct (just make sure the operator() is public)

Thaks for your help, Philippe

Answer 1

In

std::map<std::string, std::priority_queue<Person, std::vector<Person>, decltype(MyComparator)>> Familly(MyComparator);

you are calling ctor of std::map .

If you just use

std::priority_queue<Person, std::vector<Person>, decltype(MyComparator)> Familly(MyComparator);

it will work.

You need to specify std::priority_queue with lambda for every key.