具有自定义键的C ++ unordered_map导致空值

Question

I'm using a std::unordered_map with keys of Currency and values with a double of the currency price. Currency is a custom class that I made. Here is one version that I have tried:

#ifndef CURRENCY_H
#define CURRENCY_H

#include "Nameable.h"
#include <boost/uuid/uuid.hpp>
#include <boost/uuid/uuid_generators.hpp>
#include <boost/uuid/uuid_io.hpp>
#include <boost/functional/hash.hpp>
#include "BigDecimal.h"
#include <iostream>

/**
 * Represents a single currency. Can be used as keys in a map and as a general
 * identifier for determining what unit a value of money is.
 * @param name
 */
class Currency: public Nameable {
public:
    Currency(std::string name) throw(NameAlreadyTakenException);
    Currency(const Currency& orig);
    virtual ~Currency();
    virtual std::string getName();
    virtual void setName(std::string name) throw(NameAlreadyTakenException);
    inline bool operator==(const Currency& key) const {
        return this->id == key.id;
    }

    // A custom hasher that I tried using.
    struct currencyHasher
        {
        std::size_t operator()(const Currency& k) const
        {
            return boost::hash<boost::uuids::uuid>()(k.id);
        }
    };
    boost::uuids::uuid id;
private:

};
// A template specialization for Currency. 
namespace std {
    template <>
    struct hash<Currency> {
        std::size_t operator()(const Currency& k) const {
            cout<< boost::hash<boost::uuids::uuid>()(k.id)<<"\n";
            return boost::hash<boost::uuids::uuid>()(k.id);
        }
    };
}
#endif  /* CURRENCY_H */

And here is the implementation:

#include "Currency.h"

Currency::Currency(std::string name) throw(NameAlreadyTakenException) {
    this->setName(name);
    this->id = boost::uuids::random_generator()();
}

Currency::Currency(const Currency& orig) {

}

Currency::~Currency() {
}

std::string Currency::getName() {
    return this->name;
}

void Currency::setName(std::string name) throw(NameAlreadyTakenException) {
    this->name = name;
}

I tried making Currency key-compatible by implementing both suggestions given by the answer to: C++ unordered_map using a custom class type as the key . As you can see I've overridden the operator== as well as providing a custom hasher as well as specialize the template.

Despite all of this, the keys seem to be losing the values. By this I mean doubles, floats and ints get turned into 0 and string get turned into empty strings. Of course, it causes other problems with anything else that I use as a value. For example:

Currency dollar("Dollar")
std::unordered_map<Currency,int,Currency::currencyHasher> currenMap;
currenMap[dollar]=1337;
std::cout<<currenMap[dollar]<<"\n";

The output of this in the console is 0. Utilizing the template specialization doesn't work either:

std::unordered_map<Currency,int> currenMap;
currenMap[dollar]=1337;
std::cout<<currenMap[dollar]<<"\n";

produces a 0 as well...

Could the fact that Currency is a subclass of Nameable be causing problems? I'm using the boost::uuid as the hash (utilizing boost::hash<boost::uuids::uuid> to convert the id into a size_t ) I'm not sure what I'm missing, I thank you for your help.

Answer 1

The issue is with the copy constructor:

Currency::Currency(const Currency& orig) {

}

When you copy a Currency , you get a default-constructed id . When you insert a Currency into the map, it gets copied, and that copy will have a different id than the original. Thus this:

currenMap[dollar]=1337;

is effectively adding {Currency(), 1337} into the map. So when you are looking up the one with whatever id gets created for dollar , it won't be there. It's not that the value gets "zeroed out"... it's that you get a default-constructed value back.

Fixing your copy constructor should fix the problem.