overloading inserters and strange output(for '20' and '020') [duplicate]

Question

i was learning to overload '<<' in a very simple program,& during my study,i found the following surprising output of my program.

#include<iostream>
#include<conio.h>
#include<string>

using namespace std;

class student
{

int age;

 public:
student(){}
student(int a){age=a;}

friend ostream &operator<<(ostream &stream,student o); 
};

 /*operator overloaded in this block*/
 ostream &operator<<(ostream &stream,student o)
{ 
stream<<o.age;
return stream;
}

int main()
{
student ob1(20),ob2(020);   
cout<<ob1;   /*will yield 20(as desired)*/
cout<<"\n"<<ob2;     /*yielding 16(why so)*/
    _getch();
return 0;
 }

any explainations please

Answer 1

0 is the octal prefix for C++ integer literals and 20 in octal is 16 in decimal.

To explain further: If a literal starts with 0 the rest of the number will be interpreted to be in octal representation. In your example 2*8^1 + 0 * 8^0.

The syntax for integer literals is given in §2.14.2 in the most recent standard draft.

Answer 2

student ob1(20),ob2(020);

You have written 020 which in C and C++ is interpreted as an octal number (base 8) due to the zero at the start. So 020 has the value 16 in decimal.