How to set a prefix for std::ostream_iterator? - c++

Question

I would like to do something like this:
std::ofstream ch("ch_out.txt");
std::ostream_iterator< cgal_class > out( "p ", ch, "\n" );
Is this even possible? I worry because my research says no, hope it was broken. :)
The goal is to take the convex hull points produced by CGAL and write them in a file like this:
p 2 0
p 0 0
p 5 4
with this code:
std::ofstream ch("ch_out.txt");
std::ostream_iterator< Point_2 > out( "p ", ch, "\n" );
CGAL::ch_graham_andrew( in_start, in_end, out );
and the problem is that I do not want/can touch the CGAL function.

Answer 1

You have to overload the operator<< for the std::ostream class, so that it "knows" how to print an instance of your custom class.
Here is a minimal example of what I understand you want to accomplish:
#include <iostream>
#include <iterator>
#include <vector>
#include <algorithm>
class MyClass {
private:
int x_;
int y_;
public:
MyClass(int x, int y): x_(x), y_(y) {}
int x() const { return x_; }
int y() const { return y_; }
};
std::ostream& operator<<(std::ostream& os, const MyClass &c) {
os << "p " << c.x() << " " << c.y();
return os;
}
int main() {
std::vector<MyClass> myvector;
for (int i = 1; i != 10; ++i) {
myvector.push_back(MyClass(i, 2*i));
}
std::ostream_iterator<MyClass> out_it(std::cout, "\n");
std::copy(myvector.begin(), myvector.end(), out_it);
return 0;
}