Using standard library in C++, copy algorithm - c++

Question

I am trying to learn the standard library in C++ and I'm trying to run my code, but I can't succeed. What have I forgotten and what do I need to add in my class "Class" to make the program run as it should?
#include <iostream>
#include <vector>
#include <list>
#include <algorithm>
#include <iterator>
using namespace std;
class Class {
public:
Class (int ii, double dd ) : ival(ii) { dval = new double; *dval = dd; }
~Class() { delete dval; }
private:
int ival;
double *dval;
};
int main()
{
vector<Class> vec;
list<Class> list;
vec.push_back( Class(1, 2.2) );
vec.push_back( Class(2, 4.3) );
vec.push_back( Class(3, 5.7) );
ostream_iterator<const Class> classout(cout,"\n");
copy( vec.begin(), vec.end(), classout );
}
I'm basically trying to understand and use the copy-algorithm:
template
OutputIterator copy ( InputIterator first, InputIterator last,
OutputIterator result )
{
while (first!=last) *result++ = *first++;
return result;
}

Answer 1

std::ostream_iterator uses the insertion operator << to insert objects into the output stream. You must provide such an operator. An example implementation using a member function:
class Class {
public:
Class (int ii, double dd ) : ival(ii) { dval = new double; *dval = dd; }
~Class() { delete dval; }
void print(std::ostream &stream) const {
stream << ival << ' ' << *dval;
}
private:
int ival;
double *dval;
};
std::ostream& operator<< (std::ostream &stream, const Class &arg) {
arg.print(stream);
return stream;
}
As an alternative, you could also implement it by making it a friend and not creating a member function.

Answer 2

Your code has several problem. The first, which should lead to a compile error is, that your class doesn't declare a << operator for streamout, meaning that it can't be printed to a stream using a ostream_iterator. To fix that, you need to add such an operator to your class, e.g.:
class Class {
public:
Class (int ii, double dd ) : ival(ii) { dval = new double; *dval = dd; }
~Class() { delete dval; }
friend std::ostream& operator<<(std::ostream& os, const Class& c)
{
return os<<c.ival<<": "<<*c.dval;//or whatever you want your output to look like
}
private:
int ival;
double *dval;
};
However your code still contains one serious problem: A violation of the rule of three (or five/whatever if you are on c++11). Your class declares a custom destructor, but neither a custom copy constructor nor a custom assignment operator. This means that when you copy an instance of your class (such as putting it into a vector, both objects will contain the same pointer, leading to a runtime error due to double free on the destruction of the second object. To fix that you either need to declare those operations yourself or (even better) don't do resource management yourself, but use smart pointers instead (c++11 std::unique_ptr or std::shared_ptr, std::tr1::shared_ptr or a pointer from boost for pree C++11 code).