Why it causes an ERROR?

Question

I have coded a simple program in C++ on CODEBLOCK. the program is as follows:

#include <iostream>
using namespace std;

int main()
{
    int num = 09;              //ERROR: Invalid digit 9 in octal constant
    cout << num << endl;

    num = 08;             //ERROR: Invalid digit 9 in octal constant
    cout << num << endl;


    return 0;
}

This code is same as the previous one. But i have changed value and wrote num=09 instead of 015.

I'm agree that if i initialize (int num = 015), it gives output in OCTAL that is 13.

But in the above program where i tried to initialize (int num = 09 and num = 08) it gives ERROR that you can see.

First of all, I wish to know why it generates an ERROR and how?

Second is what is the logic behind it?

Please give me logical reasons with suitable examples if any.

Answer 1

Leading zeros cause integers to be interpreted as octal numbers in C/C++. 015 in octal is (1*8) + 5 = 13 in decimal.

Answer 2

Constants in 8-base are defined that way. 15₈ = 13₁₀.