Question

#include "stdafx.h"
#include "Record.h"

template<class T>//If I make instead of template regular fnc this compiles  
//otherwise I'm getting an error (listed on the very bottom) saying  
// that operator << is ambiguous, WHY?
ostream& operator<<(ostream& out, const T& obj)
{
    out << "Price: " 
        << (obj.getPrice()) << '\t'//this line causes this error
        << "Count: "
        << obj.getCount()
        << '\n';
    return out;
}

int _tmain(int argc, _TCHAR* argv[])
{
    vector<Record> v;
    v.reserve(10);
    for (int i = 0; i < 10; ++i)
    {
        v.push_back(Record(rand()%(10 - 0)));
    }
    copy(v.begin(),v.end(),ostream_iterator<Record>(cout, "\n"));
    return 0;
}

//Record class
class Record
{
    private:
        int myPrice_;
        int myCount_;
        static int TOTAL_;
    public:
        Record(){}
        Record(int price):myPrice_(price),myCount_(++TOTAL_)
        {/*Empty*/}
        int getPrice()const
        {
            return myPrice_;
        }

        int getCount()const
        {
            return myCount_;
        }
        bool operator<(const Record& right)
        {
            return (myPrice_ < right.myPrice_) && (myCount_ < right.myCount_);
        }
};

int Record::TOTAL_ = 0;

Error 2 error C2593: ‘operator <<‘ is ambiguous

Answer 1

First, you need to read the error message more carefully. As an alternative, consider breaking the statement up, something like this:

out << "Price: ";
out << (obj.getPrice());
out << "\tCount: ";
out << obj.getCount();
out << '\n';

When you do, you’ll realize that what’s really causing the problem is not where you try to print out getPrice(), but where you try to print out "Price: ".

The problem is arising because the compiler doesn’t know whether to use the normal overload to print out the string, or to use the template being defined to print it out. The latter would cause infinite recursion, and it couldn’t actually compile since it requires an object on which you can/could call getPrice and getCount to compile correctly — but it has a signature that matches, so the compiler says it’s ambiguous, and that’s the end of that.