Re: stl-vector of pointers to elements of another vector

[Guy Harrison]

Lutz Gebhardt wrote:

> Hi,
> 
> I'm trying to port a self-written C++ program to an Opteron machine with
> gcc 3.3.3 (SuSE Linux) in 64-bit mode. The program compiles and runs fine
> on SGI and SUN machines with the respective native compilers. On the Linux
> machine I have difficulties with the following piece of code (original
> code boiled down into a test snippet):
> 
> #include <iostream>
> #include <vector>
> #include <stdlib.h>
> 
> using namespace std;
> 
> int main()
> {
>   std::vector<int> testlist;
>   for (int i=0; i<10; i++)
>     testlist.push_back(i);
> 
>   std::vector<int*> testselect;
>   std::vector<int>::iterator test_itr = testlist.begin();
>   while (test_itr != testlist.end())
>     {
>       cout << *test_itr << endl;
>       if (*test_itr % 2 == 0)
>         {
>           cout << "Even number: " << *test_itr << endl;
>           testselect.push_back(test_itr);   // **** Problem statement
>           *****
>         }
>       test_itr++;
>     }
> }
> 
> My goal is to save pointers to selected elements from one STL-vector
> (testlist) into another (testselect) for later reference. With g++ I get
> the following compiler errors:
> 
> main.cpp: In function `int main()':
> main.cpp:21: error: no matching function for call to `std::vector<int*,
>    std::allocator<int*> >::push_back(__gnu_cxx::__normal_iterator<int*,
>    std::vector<int, std::allocator<int> > >&)'
> /usr/include/g++/bits/stl_vector.h:596: error: candidates are: void
>    std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = int*,
>    _Alloc = std::allocator<int*>]
> 
> Might this be caused by a missing implementation in the gcc-supplied STL
> or am I doing something very bad here which works just by chance on the
> other platforms?
> 
> Any help is greatly appreciated!

In addition to Paul's comments, you could have declared...

std::vector<std::vector<int>::iterator> testselect;

...but, unless your actual code is working upon a "const" testselect then
you're better off using indices where vector<> is concerned. Still assuming
vector<> a couple more possibilites...

int main()
{
  std::vector<int> testlist;
  for (int i=0; i<10; i++)
    testlist.push_back(i);

  std::vector<int*> testselect;
  std::vector<std::size_t> test_idx;
  std::vector<int>::iterator test_itr = testlist.begin();
  while (test_itr != testlist.end())
    {
      cout << *test_itr << endl;
      if (*test_itr % 2 == 0)
        {
          cout << "Even number: " << *test_itr << endl;
          testselect.push_back(&(*test_itr));
   test_idx.push_back(test_itr - testlist.begin());
        }
      test_itr++;
    }

 int *p(&testlist[0]);
 int **pp(&testselect[0]);
 for (std::size_t i = 0; i < test_idx.size(); i++)
  std::cout<<"i="<<i<<','
   <<(void*)(&testlist[test_idx[i]])<<','
   <<(void*)(&p[test_idx[i]])<<','
   <<(void*)(*pp++)
   <<','<<testlist[test_idx[i]]
   <<'\n'
  ;
}