I would use
A local_element;
local_element.a = 0;
local_element.b = 1;
arr.push(local_element);
local_element.a = 2;
local_element.b = 3;
arr.push(local_element);
or
const std::array<a,> arr = { A( 0, 1 ), A( 2, 3 )};
if you will write the constructor.