Question

I couldn’t find this anywhere on the web so I’m most likely is not the correct way to tackle the issue, but maybe:

I got a class which have the values A and B, I store both of these in a collective vector. Is there a way to only get access to value A from within the vector? Or would I have to create separate vectors for the values?

Someone wanted to see part of my code, so here:

class Read{
    friend ostream &operator<<(ostream &,const Read &);
    public:
        Read(char,float,float,float);
        ~Read();
    private:
        char objekt_;
        float x_, y_, r_;
};

int main(){
    vector<Read> v_read;

    while(fin >> objekt >> x >> y >> r){
        v_read.push_back(Read(objekt, x, y, r));
    }

    return 0;
}

Out from that vector I would like to access my objekt_ values in each part of the vector.

Answer 1

Given a vector ofstructs with two things in the struct we can do things like:

#include <vector>
#include <algorithm>
#include <boost/bind.hpp>
#include <iterator>
#include <iostream>

struct test {
  int a;
  double b;
};

int main() {
  std::vector<test> vec = {{0, 0.1}, {1, 0.2}};

  // Copy the a's to another iterator
  std::transform(vec.begin(), vec.end(),
                 std::ostream_iterator<int>(std::cout, "\n"),
                 boost::bind(&test::a,_1));

  // and the b's to another iterator
  std::transform(vec.begin(), vec.end(),
                 std::ostream_iterator<double>(std::cout, "\n"),
                 boost::bind(&test::b,_1));

  // or just copy them into another vector
  std::vector<int> veca;
  veca.reserve(vec.size());
  std::transform(vec.begin(),vec.end(),
                 std::back_inserter(veca),
                 boost::bind(&test::a,_1));
}

To work with only part of each struct for all the entries in a given std::vector.