Initialize members of a class in C++ - c++

Question

I have a class with the following declaration:
class OrderBook {
std::string OrderBookType;
std::map<double, vector<float*>> OrderBookData;
std::unordered_map<int, float*> OrderPool;
public:
OrderBook(string);
}
I have 2 questions regarding the class initialization:
I would like to specify the comparator for the OrderBook map based on OrderBookType.
Is the following valid/good programming practice?
OrderBook::OrderBook(string bookType) {
OrderBookType = bookType;
if (bookType == 'B') {
OrderBookData = std::map<float, vector<float*>, std::greater<float>>
}
else {
OrderBookData = std::map<float, vector<float*>, std::less<float>>
}
OrderPool;
}
How do I initialize an empty map for OrderPool? To make sure OrderPool is an empty unordered map, do I have to do the following:
OrderPool = std::unordered_map<int, float*>()
or is OrderPool; sufficient?

Answer 1

To 1.
No, it's not valid.
The types
std::map<float, vector<float*>, std::greater<float>>
and
std::map<float, vector<float*>, std::less<float>>
are different. You can't change the types at runtime.
IMHO the best approach is to create an own comparator that expects an argument and is passed to the constructor of the map (as mentionend in the comments of the question).
class Comp {
public:
Comp(bool greater) : g(greater) {}
bool operator()(double lhs, double rhs) const {
if (g) return lhs > rhs;
return lhs < rhs;
}
private:
bool g;
};
struct OrderBook {
std::string OrderBookType;
std::map<double, std::vector<float*>, Comp> OrderBookData;
std::unordered_map<int, float*> OrderPool;
public:
OrderBook(const std::string &);
};
OrderBook::OrderBook(const std::string &bookType) : OrderBookType(bookType), OrderBookData(Comp(bookType == "B")) {}
To 2.
std::unordered_map<int, float*> OrderPool; will be initialized as empty unordered map. The line
OrderPool;
does nothing.
You don't need to (and technically speaking you can't) initialize it again. If you want to clear the map you can use std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::clear
You could also create a new object and assign it with
OrderPool = std::unordered_map<int, float*>()
but I prefer
OrderPool.clear();