Why numbers starting from 0 is treated as octal in cpp?

Question

For ex: int a=010;
cout<<a;
Outputs 8

Answer 1

Such prefix are used for integer literals, to signal the intended base for the integer literal. There are some well known integer literal prefix commonly used. Among which are:
0b - Prefix for binary integer literal
0 - Prefix for octal integer literal
0x - Prefix for hexadecimal integer literal
Learn more about integer literals, prefix and suffix here 👇
https://www.tutorialspoint.com/cprogramming/c_constants.htm