what is the reason for the difference between setprecision (12) and setprecision (012) for example,in c++?

Question

In c++ when you write setprecision (12) for example, 12 is in the base of 10 but when you write it like setprecision (012) it is an octal number,why?

Answer 1

Because constants with leading zeros (other than leading 0x) are always octal:

An octal integer literal (base eight) begins with the digit 0 and consists of a sequence of octal digits.

C++ draft standard (n1905) §2.13.1

It has nothing at all to do with setprecision.

Answer 2

In C++, if an integer literal begins with a 0 digit (and that 0 is not followed by a x), the following digits are treated as octal digits.

Answer 3

Because that's how it worked in C. Back when C was designed, octal numbers were in frequent use, so they put in a notation for them. Currently, it's rarely helpful and mostly confusing.