c++ - Do Vectors resize automatically? -

i sorry asking such beginner question finding contradictory information online. ask @ university out until february next year.

do vectors resize automatically? or need check current size periodically , resize when need more room. looks resizing me automatically i'm not sure if feature or compiler waving magic wand.

if use push_back or insert, yes vector resizes itself. here demo:

#include<iostream> #include<vector>  using namespace std;  int main() {     vector < int > a;     a.push_back(1);     a.push_back(2);     a.push_back(3);     (int value : a) {         cout << value << " ";     }     cout << endl << "current size " << a.size() << endl;     return 0; } 

it gives output as:

1 2 3 current size 3 

remember if a[3] = 5. not resize vector automatically.
can manually resize vector if want. demo append following code above code.

a.resize(6); (int value : a) {     cout << << " "; } cout << endl << "current size " << a.size() << endl; 

now output:

1 2 3 current size 3 1 2 3 0 0 0 current size 6 

it think got answer.