Learning immediately: https://edu.csdn.net/course/play/20589/257754?utm_source=blogtoedu
1. implicit conversions:
(1) users do not need a control, the compiler automatically recognize
(2) the type of small-scale type of assignment to a wide range of inside, the compiler will automatically help us convert
int a = 15;
double b = a;
2. Display conversion: we need to specify the type of goal conversion