std::vector a { 1, 2, 3 }; // allocate 3 elements with values 1, 2, and 3
std::vector b { 3 }; // allocate 1 element with value 3
std::vector<int> c ( 3 ); // allocate 3 elements with values 0, 0, and 0
std::vector<int> d ( 3, 4 ); // allocate 3 elements with values 4, 4, and 4