原完整教程链接：6.11a References and const
1.
/*
Unlike references to non-const values, which can only be initialized
with non-const l-values, references to const values can be initialized
with non-const l-value, const l-values, and r-values.
*/
int x = 5;
const int &ref1 = x; // okay, x is a non-const l-value
const int y = 7;
const int &ref2 = y; // okay, y is a const l-value
const int &ref3 = 6; // okay, 6 is an r-value
