View Compilation Error

0_0_35857054_15260.cpp: In function 'int main()':
0_0_35857054_15260.cpp:7:1: error: 'optional' was not declared in this scope
 optional<int> x(a), y(b);
 ^
0_0_35857054_15260.cpp:7:10: error: expected primary-expression before 'int'
 optional<int> x(a), y(b);
          ^