Constructor Initialiser List

Ok, I'm having a weird problem with getting my code to compile. The compiler (g++) keeps telling me that the "copy constructor" is private and gives me "error: within this context". Here's my following code:

Code:

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#include <iostream>
#include <cmath>

class Crypt {
        private:
                std::istream strm;
                int n;
                int p;
                int k;        // want to find this value
        public:
                // problem is here
                Crypt(std::istream &is): strm(is), n(0), p(0), k(0)
                {
                }

                void getpair();
                void print()
                {
                        std::cout << "n: " << n << " p: " << p << " k: " << k << std::endl;
                }
                       
                int calculate();
};

int main(int argc, char **argv)
{
        Crypt cval(std::cin);        /* apply default constructor */
        cval.getpair();
        std::cout << cval.calculate() << std::endl;
        return 0;
}

inline void Crypt::getpair()
{
        /* could use some improvement by checking */
        int val1;
        std::cin >> val1;
        n = val1;
        std::cin >> val1;
        p = val1;
}

inline int Crypt::calculate()
{
        int val = 0;
        /* need to fix casts/variable types */
        for (k = 0; k < p; k++) {
                val = static_cast<int> (std::pow(static_cast<double>(k), static_cast<double>(n)));
                if (val == p) {
                        return k;
                }
        }
        return -1;
}

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What I'm trying to do in this code segment:

Code:

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                Crypt(std::istream &is): strm(is), n(0), p(0), k(0)
                {
                }

---

Is to initialise strm to the value sent in by is. I'm not sure why it's complaining about a copy constructor since I used a reference as the argument for the constructor which shouldn't copy anything. And I also used the constructor initialisation list instead of "copying" it in the body block. I'm literally stuck here with no clue on what's going on. Any help would be great :)

EDIT: Here's the full compiler warning:

Code:

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crypt113.cpp: In copy constructor `std::basic_ios<char, std::char_traits<char> >::basic_ios(const std::basic_ios<char, std::char_traits<char> >&)':
/usr/lib/gcc/i486-slackware-linux/3.4.5/../../../../include/c++/3.4.5/bits/ios_base.h:781: error: `std::ios_base::ios_base(const std::ios_base&)' is private
crypt113.cpp:12: error: within this context

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> I used a reference as the argument for the constructor which shouldn't copy anything.
No, but you do try and make an actual copy of it with
strm(is)
This too needs to be a reference (your member variable)

Hmm I'm still slightly confused, wouldn't "strm(is)" call the constructor which takes a single std::istream argument (i.e. direct initialisation)? At the moment I can't visualise it properly (still new to C++) - I can't see why strm needs to be a reference and not an actual object. Wouldn't strm act like a "normal" variable getting initialised like:

Code:

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...
std::istream samp(std::cin);
...

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Or would that not work? *Argh* got to revise more :(

What if you re-design your class so that you pass the std::istream by reference to whichever member function needs to use it? An example of this would be overloading operator>>

samp in your class is a stream varable, streams are, in general, non-copyable. The normal way to design something like this is to have a friend >> operator.

Code:

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class Crypt {
...
friend std::istream & operator >> (std::istream &, Crypt &);
};
std::istream & operator >> (std::istream &is, Crypt &c) {
    return is >> c.n >> c.p;
}
int main() {
    Crypt cval;
    if(std::cin >> cval) {
        cval.calculate();
        ...
    }
}

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You may also want to call calculate() from within the >> operator, so that k is always valid. You may also want to read the values into local varables and only change the target Crypt if all input operations succeed.

Just make strm a reference, like Salem said:

Code:

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class Crypt {
        private:
                std::istream &strm;

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Note that this makes your class non-assignable. If you want it assignable, you must store a pointer. Also note that the Crypt object must not outlive the stream you pass to the constructor.

Finally, you should make Crypt's constructor explicit, to prevent implicit conversion from an istream to a Crypt, which would make no sense:

Code:

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public:
                explicit Crypt(std::istream &is): strm(is), n(0), p(0), k(0)

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