What is the function of auto and &(x) ? (they work the same way in this snippet of code below)

Question

vector<int> v{3,1,4,1,5,9};

for(int &x: v) cout<< x << ' ' ;
cout<< '\n';
for(auto x: v) cout<< x << ' ';

Answer 1

The "auto" keyword was introduced in C++11 so that you don't have to write

std::vector<std::pair<std::vector<int>, int>>

and other lengthy types every time you need to iterate over it, or to declare one with initialization.

The compiler would deduce what datatype the variable/object should be. There are limitations though:

auto x = 5; // ok

auto x; // error
x = 5;

As for &, it's used to tell the compiler that x is a reference of the original elements in v, not merely a temporary variable storing the values of elements in v. Altering x will alter the elements in v.