problem passing in istream argument to a class constructor.

Question

I have the following code in my header file:

class Factovisors {

    public:
        Factovisors(std::istream& strm):strm_(strm)
        {

        }
        void run()
        {
            unsigned int n,m;
            while (!strm_.eof()) {
            strm_ >> n >> m;

            if (isFact(n,m))
                std::cout << m << " divides " << n << "!\n";
            }
        }

        std::istream strm_;

};

My .cpp file has the following code.

 std::ifstream strm("factovisor.test");

    Factovisors   facto(strm);

    facto.run();

    strm.close();

The error my compiler gives me is:

std::ios::basic_ios(const std::ios &) is not accessible from
std::istream::basic_istream(const std::istream &)

I imagine I am missing something really obvious. So any help would be greatly appreciated.

Answer 1

The problem is that istream is an "interface". It has pure virtual functions, so it doesn't make sense to have a copy of it. What you might do is to keep a reference to the passed stream:

std::istream& strm_;

strm_ could be ifstream or istringstream or any input stream derived from istream.

Answer 2

You can't copy-construct a stream because the base-class ios has its copy ctor private. Try making the stream member a reference, rather than a standalone object.

Answer 3

You are trying to store a copy of the stream. This will not work, since streams are not copyable. Best you can do is store a reference or a pointer.

However, if only one method is going to use the stream, just pass a reference to this method.

Other problems:

        while (!strm_.eof()) {
        strm_ >> n >> m;

        if (isFact(n,m))

Eof is set when an attempt to read data fails because of this. As it is you are bound to read the last entry twice. Instead:

while (strm >> n >> m )
    if (isFact(n, m)