Using a C++ iterator to read a list from a file?

Question

I am trying to read/deserialize a list of elements from a file (and then filter out some of them). It is a useful approach to use an iterator for this purpose?

My current try is

#include <boost/iterator/iterator_adaptor.hpp>
class ReadIterator : public boost::iterator_adaptor<ReadIterator, Elem *, boost::single_pass_traversal_tag>
{
public:
    explicit ReadIterator(const char *filename) : reader(filename)  {}

private:
    friend class boost::iterator_core_access;

    void increment() {
         this->base_reference() = reader.readNext();
    }

    Reader reader;
};

This does not properly deallocate memory (e.g., readNew returns a pointer to a new Elem), what is the right way to do this? Also, how would one actually use such an iterator, how can the end be determined? Or is there a better approach than using an iterator?

Answer 1

The easy way to do this is to use the std::istream_iterator

std::vector<YourObjectClass>    data;

std::remove_copy_if(std::istream_iterator<YourObjectClass>(file),
                    std::istream_iterator<YourObjectClass>(),
                    std::back_inserter(data),
                    YourFilter
                   );

The standard algorithm copies objects (of type YourObjectClass) from the input file and places them into the vector data if the filter functor returns true.

The only conditions are:

• YourObjectClass must have an input stream operator
• YourFilter must overload the operator() for objects of YourObjectClass or be a function that takes a parameter of type YourObjectClass.

Simple Working Example:

• My object is a line.
• Filter out line(s) that start with the letter 'A'

Exmpale:

#include <vector>
#include <string>
#include <fstream>
#include <iterator>
#include <algorithm>

struct Line
{
    std::string   data;
};
std::istream& operator>>(std::istream& stream, Line& line)
{
    return std::getline(stream, line.data);
}
struct AFilter
{
    bool operator()(Line const& line) const
    {
        return line.data.size() > 0 && line.data[0] == 'A';
    }
};

int main()
{
    std::ifstream       file("Plop");
    std::vector<Line>   data;

    std::remove_copy_if(std::istream_iterator<Line>(file),
                        std::istream_iterator<Line>(),
                        std::back_inserter(data),
                        AFilter()
                       );
}