segmentation fault in stl vector.put_back

[purso01]

I get a segmentation fault error when I compile the following program.
The v2 vector contains ponters to float values in the array v.

#include <iostream>  
#include <fstream>
#include <vector>
#include <algorithm>
using namespace std;
int main (int argc, char *argv[])
{ float a = 3, b = 2, c = 1;
  float v[3];
  v[0] = a;  v[1] = b;  v[2] = c;  
  vector<float*> v2(3); 
 // v2[0] = v+0;  v2[1] = v+1;   v2[2] = v+2;  // runs fine, but 
  v2[0] = v+0;  v2[1] = v+1;   v2.push_back(v+2); // ****ERROR****  
  cout << *(v2[0]) << "  " << *v2[1] << "  " << *v2[2] << endl;
return 0;
}

If I remove the comment slashes from the line above the errorous line the errorous line also runs fine, but it does not run as such. Odd? And
if I comment the push_back line the commented line runs fine.

Can't I use put_back in this way? I cannot see the mistake by myself, so I would apriciate very much any hints that would explain the error.

purso

[horace1]

you create a vector with 3 elements and you set up [0] and [1] with the address of v+0 and v+1

  vector<float*> v2(3); 
 // v2[0] = v+0;  v2[1] = v+1;   v2[2] = v+2;  // runs fine, but 
  v2[0] = v+0;  
  v2[1] = v+1;   
 

you the call push back which ADDS an element of the end of v2 (it now has 4 elements), i.e. element v2[3] contains the address of v+2

  v2.push_back(v+2); // ****ERROR****  
 

so when you try to print

  cout << *(v2[0]) << "  " << *v2[1] << "  " << *v2[2] << endl;
 

v2[2] does no contain a valid address so you get a segmentation fault
try printing *v2[3] and it should work

  cout << *(v2[0]) << "  " << *v2[1] << "  " << *v2[3] << endl;
 

[purso01]

Thank you, horace1.

Now, i understand what was the mistake. I assumed that push_back only adds items to a vector when it is needed. I thought that as i had only two members added into the vector, the third should go to the remaining empty slot. That was wrong.

The revised program runs fine.

#include <iostream>  
#include <fstream>
#include <vector>
#include <algorithm>
using namespace std;
int main (int argc, char *argv[])
{ float a = 3, b = 2, c = 1;
  float v[3];
  v[0] = a;  v[1] = b;  v[2] = c;  
  vector<float*> v2(2);  // this is now (only) 2
  v2[0] = v+0;  v2[1] = v+1;   v2.push_back(v+2); // runs fine!  
  cout << *(v2[0]) << "  " << *v2[1] << "  " << *v2[2] << endl;
return 0;

purso