C++ vector do not keep value received from loop -

i'm writing program takes value file. value require modified @ further moment, store them in vector.

basically program is:

• read whole file while loop.
• if line contain required parameter array, store every value array in vector using loop.

problem is:

as loop exited, vector lost everything. declared vector outside while loop read file, should not happen. 1 know going on?

example file:

<polylist count="34">     <input semantic="vertex" source="#cylinder-mesh-vertices" offset="0"/>     <input semantic="normal" source="#cylinder-mesh-normals" offset="1"/>     <vcount>4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 32 4 4 32 </vcount>     <p>0 0 1 0 3 0 2 0 2 1 3 1 5 1 4 1 4 2 5 2 7 2 6 2 6 3 7 3 9 3 8 3 8 4 9 4 11 4 10 4 10 5 11 5 13 5 12 5 12 6 13 6 15 6 14 6 14 7 15 7 17 7 16 7 16 8 17 8 19 8 18 8 18 9 19 9 21 9 20 9 20 10 21 10 23 10 22 10 22 11 23 11 25 11 24 11 24 12 25 12 27 12 26 12 26 13 27 13 29 13 28 13 28 14 29 14 31 14 30 14 30 15 31 15 33 15 32 15 32 16 33 16 35 16 34 16 34 17 35 17 37 17 36 17 36 18 37 18 39 18 38 18 38 19 39 19 41 19 40 19 40 20 41 20 43 20 42 20 42 21 43 21 45 21 44 21 44 22 45 22 47 22 46 22 46 23 47 23 49 23 48 23 48 24 49 24 51 24 50 24 50 25 51 25 53 25 52 25 52 26 53 26 55 26 54 26 54 27 55 27 57 27 56 27 56 28 57 28 59 28 58 28 58 29 59 29 61 29 60 29 3 30 1 30 63 30 61 30 59 30 57 30 55 30 53 30 51 30 49 30 47 30 45 30 43 30 41 30 39 30 37 30 35 30 33 30 31 30 29 30 27 30 25 30 23 30 21 30 19 30 17 30 15 30 13 30 11 30 9 30 7 30 5 30 62 31 63 31 1 31 0 31 60 32 61 32 63 32 62 32 0 33 2 33 4 33 6 33 8 33 10 33 12 33 14 33 16 33 18 33 20 33 22 33 24 33 26 33 28 33 30 33 32 33 34 33 36 33 38 33 40 33 42 33 44 33 46 33 48 33 50 33 52 33 54 33 56 33 58 33 60 33 62 33</p> </polylist> 

c++ code:

#include <iostream> #include <string> #include <fstream> #include <sstream> #include <iomanip> #include <algorithm> #include <math.h> #include <vector> using namespace std; int main(){     vector<int> meshprimlengtharray;     ifstream readcollada("mycollada.dae", ios::in);     string currentline("");     while(getline(readcollada, currentline)){ //loop read file         if(currentline.find("<polylist") != std::string::npos){ //the given line tell on many value next array contain, can reserve required space in vector.                 string numberofvcount(currentline.substr(currentline.find("count=\"")+7, currentline.find("\">")-(currentline.find("count=\"")+7)));                 meshprimlengtharray.reserve(atoi(numberofvcount.c_str()));         }         else if((currentline.find("<vcount>") != std::string::npos) && (currentline.find("</vcount>") != std::string::npos)){             currentline = currentline.substr(currentline.find("<vcount>")+8, currentline.find("</vcount>")-(currentline.find("<vcount>")+8));             istringstream vcountarray(currentline);             if(vcountarray){                 for(int i(0); i<(int)meshprimlengtharray.capacity(); i++){                     vcountarray >> meshprimlengtharray[i];                     cout << " " << meshprimlengtharray[i] << ","; //show value have been inserted vector.                 }                 cout << endl << "meshprimlengtharray.size() = " << meshprimlengtharray.size() << endl; //show unknow reason vector have been cleared.             }         }     }     cout << endl << "press enter continu" << endl;     cin.get();     cin.ignore();     return 0; } 

vcountarray >> meshprimlengtharray[i] 

expects elements constructed, aren't. std::vector::reserve not that, std::vector::resize does. code exhibits undefined behavior.

you might use temporary , std::vector::push_back (that goes std::vector::reserve, makes potentially faster - avoids reallocations).

finally, difference between std::vector::size , std::vector::capacity.

if use operator[], use i < meshprimlengtharray.size() loop condition.

^{prefer using right types - size_t in case, instead of int , c-style cast!
of course, loop can translated range-based loop.}