How to pass a multimap into function

Question

I have a pretty simple question. Im just learning Maps and multimaps and want to know how to pass them into a function. Ive got most of my mind wrapped around multimaps but would like a quick example on how to pass them into a void function.

int main()
{
multimap<string,int> movies;


movies.insert(pair<string,int>("Happy Feet",6));
movies.insert(pair<string,int>("Happy Feet",4));
movies.insert(pair<string,int>("Pirates of the Caribbean",5));
movies.insert(pair<string,int>("Happy Feet",3));
movies.insert(pair<string,int>("Pirates of the Caribbean",4));
movies.insert(pair<string,int>("Happy Feet",4));
movies.insert(pair<string,int>("Flags of out Fathers",4));
movies.insert(pair<string,int>("Gigli",4));

cout<<"There are "<<movies.count("Happy Feet")<<" instances of "<<"Happy Feet"<<endl;
cout<<"There are "<<movies.count("Pirates of the Caribbean")<<" instances of "<<"Pirates of the Caribbean"<<endl;
cout<<"There are "<<movies.count("Flags of out Fathers")<<" instances of "<<"Flags of out Fathers"<<endl;
cout<<"There are "<<movies.count("Gigli")<<" instances of "<<"Gigli"<<endl;



system("PAUSE");
calculateAverage(movies);  // this is where im getting errors such as no conversions
return 1;
}
void calculateAverage(multimap<string,int> *q)
{
// this function wont calculate the average obviously. I just wanted to test it
int averageH;
int averageP;
int averageF;
int averageG;

averageH = (q->count("Happy Feet"));
averageP = (q->count("Happy Feet"));
averageF = (q->count("Happy Feet"));
averageG = (q->count("Happy Feet"));


};

Answer 1

Why pass by pointer? I think it is better to pass a reference (if the map shall be modified within the function) or reference to const otherwise

void calculateAverage(const multimap<string,int> & q)
{
// this function wont calculate the average obviously. I just wanted to test it
int averageH;
int averageP;
int averageF;
int averageG;

averageH = (q.count("Happy Feet"));
averageP = (q.count("Happy Feet"));
averageF = (q.count("Happy Feet"));
averageG = (q.count("Happy Feet"));
};

Answer 2

Pass by reference:

void calculateAverage(const multimap<string,int> & q)

But then passing pointer is not that bad. It's just that syntax doesn't look good.

If you choose to pass pointer, then at the calling site, you've to use this syntax:

calculateAverage(&movies);

Answer 3

It seems to me more "in the spirit of the STL" to pass to iterators, movies.begin() and movies.end() to the calculateAverage function. For example:

calculateAverage(movies.begin(),movies.end());

with the following defined:

typedef multimap<string,int>::const_iterator MapIt;
void calculateAverage(const MapIt &begin, const MapIt &end)
{
...
}

Answer 4

You are trying to pass a value of type multimap<string,int> as a pointer to that type, i.e. multimap<string,int>*. Either change the function signature to void calculateAverage(const multimap<string,int>& q) and modify its code accordingly (replace -> with .), or call it like this: calculateAverage(&movies).