segmentation fault in stl vector.put_back

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

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 #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

Feb 19 '07 #1

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horace1

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Expert 1GB

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

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  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

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

so when you try to print

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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

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

Feb 19 '07 #2

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.

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 #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

Feb 19 '07 #3

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