for(auto a:b) usage

In for(auto a:b), b is a container. The effect is to use a to traverse and obtain each value in the b container, but a cannot affect the elements in the b container.

For (auto & a: b) is added a reference symbol, you can assign a value to the content in the container, you can fill the content of the container b by assigning a value to a.

int arr[10];
for(int i=0;i<10;i++)
{
    
    
  arr[i]=i;
}
for(auto &a:arr)
{
    
    
  std::cout << a;
}

Output results 1-9