vector of existing objests

Question

I have some objects (they are vectors in this example) and I want them to be stored in a vector but don\'t know how to declare it properly. My code is:

vector<         


        

Answer 1

You had to store vector& or vector* in your Tables. The latter should work but I tried to manage the first. Concerning this I found SO: Why can't I make a vector of references?. Thus, I tried something else: std::reference_wrapper and it seems to be a (/the) solution:

#include <functional>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

int main(int, char**)
{
  // build contents of tables
  vector<string> a { string{"A 0"}, string{"A 1"}, string{"A 2"} };
  cout << "&a: " << hex << (size_t)&a << endl;
  vector<string> b { string{"B 0"}, string{"B 1"}, string{"B 2"} };
  cout << "&b: " << hex << (size_t)&b << endl;
  vector<std::reference_wrapper<vector<string> > > tables {
    a, b
  };
  // print contents of tables
  for (size_t i = 0, n = tables.size(); i < n; ++i) {
    const vector<string> &tbl = tables[i];
    cout << "&tables[" << i << "]: " << hex << (size_t)&tbl << endl;
    for (size_t j = 0, m = tbl.size(); j < m; ++j) {
      cout << "tables[" << i << "][" << j << "]: '"
        << tbl[j] << "'" << endl;
    }
  }
  // append another string to a
  { vector<string> &tbl = tables[0];
    tbl[1] = string("A 1 modified");
    tbl.push_back(string("A 3"));
    tbl.emplace_back(string("A 4"));
  }
  // print contents of a
  for (size_t i = 0, n = a.size(); i < n; ++i) {
    cout << "a[" << i << "]: '" << a[i] << "'" << endl;
  }
  // append another table
  vector<string> c { string("C 0"), string("C 1"), string("C 2") };
  cout << "&c: " << hex << (size_t)&c << endl;
  /* my 1st guess:
  tables.emplace_back(std::ref(c));
   * but even this works:
   */
  tables.emplace_back(c);
  // print contents of tables
  for (size_t i = 0, n = tables.size(); i < n; ++i) {
    const vector<string> &tbl = tables[i];
    cout << "&tables[" << i << "]: " << hex << (size_t)&tbl << endl;
    for (size_t j = 0, m = tbl.size(); j < m; ++j) {
      cout << "tables[" << i << "][" << j << "]: '"
        << tbl[j] << "'" << endl;
    }
  }
  // done
  return 0;
}

Compiled this with gcc on cygwin:

$ gcc --version
gcc (GCC) 5.4.0
Copyright (C) 2015 Free Software Foundation, Inc.

$ g++ -std=c++11 -o testVecRef testVecRef.cc 

$ ./testVecRef
&a: 61cbec
&b: 61cbe0
&tables[0]: 61cbec
tables[0][0]: 'A 0'
tables[0][1]: 'A 1'
tables[0][2]: 'A 2'
&tables[1]: 61cbe0
tables[1][0]: 'B 0'
tables[1][1]: 'B 1'
tables[1][2]: 'B 2'
a[0]: 'A 0'
a[1]: 'A 1 modified'
a[2]: 'A 2'
a[3]: 'A 3'
a[4]: 'A 4'
&c: 61cbc8
&tables[0]: 61cbec
tables[0][0]: 'A 0'
tables[0][1]: 'A 1 modified'
tables[0][2]: 'A 2'
tables[0][3]: 'A 3'
tables[0][4]: 'A 4'
&tables[1]: 61cbe0
tables[1][0]: 'B 0'
tables[1][1]: 'B 1'
tables[1][2]: 'B 2'
&tables[2]: 61cbc8
tables[2][0]: 'C 0'
tables[2][1]: 'C 1'
tables[2][2]: 'C 2'

Compiled and started with MS VS2013:

&a: cfba2ff2b8
&b: cfba2ff378
&tables[0]: cfba2ff2b8
tables[0][0]: 'A 0'
tables[0][1]: 'A 1'
tables[0][2]: 'A 2'
&tables[1]: cfba2ff378
tables[1][0]: 'B 0'
tables[1][1]: 'B 1'
tables[1][2]: 'B 2'
a[0]: 'A 0'
a[1]: 'A 1 modified'
a[2]: 'A 2'
a[3]: 'A 3'
a[4]: 'A 4'
&c: cfba2ff508
&tables[0]: cfba2ff2b8
tables[0][0]: 'A 0'
tables[0][1]: 'A 1 modified'
tables[0][2]: 'A 2'
tables[0][3]: 'A 3'
tables[0][4]: 'A 4'
&tables[1]: cfba2ff378
tables[1][0]: 'B 0'
tables[1][1]: 'B 1'
tables[1][2]: 'B 2'
&tables[2]: cfba2ff508
tables[2][0]: 'C 0'
tables[2][1]: 'C 1'
tables[2][2]: 'C 2'
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