c++ - Declaration of Vectors -

vectors size dynamically, why giving seg fault:

#include <iostream> #include <string> #include <vector>  using namespace std;  int main(){     vector<int> vectorofints;      vectorofints[0] = 3; } 

what i'm trying declare vector in class.

#include <iostream> #include <string> #include <vector>  using namespace std;  class directory{     public:         string name;         int maxindex;         vector<directory> subdirectories;          void addsubdirectory(string x){             directory newsubdirectory(x);             subdirectories[maxindex++] = newsubdirectory;         }          directory(string x){             name = x;             maxindex = 0;         } };  int main(){     directory root("root");     root.addsubdirectory("games"); } 

but gives seg fault.

vectors don't resize entirely automatically. use push_back or resize change size of vector @ run-time, vector not automatically resize based on index use--if index beyond current size, undefined behavior.

in demo code, this:

vector<int> vectorofints(1);  vectorofints[0] = 3; 

alternatively, since you're adding 3 end of existing data (or nonexistent data, in case) use push_back (or emplace_back):

vector<int> vectorofints; vectorofints.push_back(3); 

it looks same basic approach work real code well. simplifies things bit, since don't need explicitly track maxindex you've done.