Question

Lets say I wanted to make a vector of objects and another vector of pointers to those objects (I cannot use dynamic memory). The way I would do it is in the following example.

#include <iostream>
#include <vector>

using namespace std;

class Foo {
public:
  int bar;
  Foo(int x) : bar(x) {
  }
};

int main () {
  vector<Foo> foos;
  vector<Foo*> pFoos;
  for (int i = 0; i < 10; i++) {
    Foo foo(i);
    foos.push_back(foo);
    pFoos.push_back(&foos.back());
  }

  for (int i = 0; i < 10; i++) {
    cout << foos[i].bar << endl;
    cout << pFoos[i]->bar << endl;
  }
}

I think this should work because foos stores a copy of the object, and then I store the reference to the copy (because the original foo will be undefined, so I shouldn’t store a reference to that). But this is what I get:

0
36741184
1
0
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9

The first to numbers from pFoos are wrong. Furthermore, the large number changes every time. I don’t see anything that would cause this undefined behavior. Could someone tell me what I’m doing wrong?

Answer 1

Adding items to a vector invalidates all previous iterators. Calling push_back on the vector may make the pointers you got from it previously invalid if the vector needs to reallocate its internal storage.

If you knew that you were never going to grow the vector again then this would work:

for (int i = 0; i < 10; i++) {
  foos.push_back(Foo(i));
}

for (int i = 0; i < 10; i++) {
  pFoos.push_back(&foos[i]);
}

or as commented by rodrigo:

foos.reserve(10)

for (int i = 0; i < 10; i++) {
  Foo foo(i);
  foos.push_back(foo);
  pFoos.push_back(&foos.back());
}

for (int i = 0; i < 10; i++) {
  cout << foos[i].bar << endl;
  cout << pFoos[i]->bar << endl;
}