overloading inserters and strange output(for '20' and '020')

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

Pass student by const reference. Otherwise you are making a copy when you pass it into the function. `ostream &operator<<(ostream &stream,student **const&** o)` — Martin York, Aug 11 '11 at 13:59
Hope you got the answer. For better performance declare it as: `friend ostream &operator<<(ostream &stream,const student& o);` — Ajay, Aug 11 '11 at 14:47

pmr · Accepted Answer · 2011-08-11T14:08:05.077

11

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.

edited Aug 11 '11 at 14:08

answered Aug 11 '11 at 13:51

pmr

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score 5 · Answer 2 · answered Aug 11 '11 at 13:52

5

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.

answered Aug 11 '11 at 13:52

jcoder

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score 3 · Answer 3 · edited May 23 '17 at 12:02

3

You will find the answer here:

How does C Handle Integer Literals with Leading Zeros, and What About atoi?

You prefixed the literal with a zero, which means treat the number as octal and 20oct is 16dec.

edited May 23 '17 at 12:02

Community

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answered Aug 11 '11 at 13:52

Skizz

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score 2 · Answer 4 · answered Aug 11 '11 at 13:53

2

020 is 20 in octal, just as 0x20 is 20 in hex. Remove the preceding 0 and it should work.

answered Aug 11 '11 at 13:53

user786653

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score 1 · Answer 5 · answered Aug 11 '11 at 13:53

1

The leading 0 specifies that the it is an octal number (base 8) so 20 in octal is 16.

answered Aug 11 '11 at 13:53

love_me_some_linux

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score 1 · Answer 6 · answered Aug 11 '11 at 13:55

1

It's interpereting the 020 as an octal. 2*8=16. Try switching it to a 3, itll show 24 for 030. If you put 0x20 itll go to 32 as its doing hexadecimal. :)

answered Aug 11 '11 at 13:55

John Humphreys

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overloading inserters and strange output(for '20' and '020')

6 Answers6