boost::multi_index_container with random_acces and hashed_unique

Question

I'am trying to create container with both index and key access with unique keys: so here is my code:

#include <QString>
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/random_access_index.hpp>
#include <boost/multi_index/mem_fun.hpp>

class MyClass{
public:
    MyClass( const QString& name ): mName( name ){}
    QString name    ( ) const { return mName; }
    void    setName ( const QString& name ) { mName = name ;}
private:
    QString mName;
};


using namespace boost::multi_index;
typedef multi_index_container<
    MyClass,
    indexed_by<
        random_access<tag<struct RandomIndex>>,
        hashed_unique<
            tag<struct NameIndex>,
            const_mem_fun<MyClass, QString, &MyClass::name>
        >
    >
> Table;

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Table table;
    for (MyClass v : { "first", "second", "third" }){
        table.push_back(v);
    }

    return a.exec();
}

When I try to compile it I have an error boost_1_55_0\boost\functional\hash\extensions.hpp:269: error: C2664: 'size_t boost::hash_value(std::type_index)' : cannot convert argument 1 from 'const QString' to 'std::type_index' No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

As I can see boost doesn't know nothing abt Qt class QString, but I am fail to find how to fix this problem. Can someone explain how to fix my code?

Answer 1

You need to tell Boost how to hash QString.

Simplest way would seem:

namespace boost {
    template <> struct hash<QString> {
        size_t operator()(QString const& v) const {
            return qHash(v);
        }
    };
}

Complete working example:

#include <QtCore/QString>
#include <QtCore/QHash>
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/random_access_index.hpp>
#include <boost/multi_index/mem_fun.hpp>

class MyClass{
public:
    MyClass( const QString& name ): mName( name ){}
    QString name    ( ) const { return mName; }
    void    setName ( const QString& name ) { mName = name ;}
private:
    QString mName;
};

namespace boost {
    template <> struct hash<QString> {
        size_t operator()(QString const& v) const {
            return qHash(v);
        }
    };
}


using namespace boost::multi_index;
typedef multi_index_container<
    MyClass,
    indexed_by<
        random_access<tag<struct RandomIndex>>,
        hashed_unique<
            tag<struct NameIndex>,
            const_mem_fun<MyClass, QString, &MyClass::name>
        >
    >
> Table;

#include <QtGui/QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Table table;
    for (QString v : { "first", "second", "third" }){
        table.push_back(MyClass(v));
    }

    return a.exec();
}