Are there any problems with the c++ code

Question

vector<int> vec1 = {5, 2, 5, 2, 0};
vector<int> vec2;
for(int i = 0; i < vec1.size(); ++i)
vec2[i] = vec1[i];

Answer 1

Moreover, I saw the second vector's size isn't defined. So, in your loop, write :
vec2.push_back(vec1[i])

Answer 2

you need to include vector library first to work with vectors:
#include <vector>
Then everything will be fine!☺