Getting invalid initialization of reference of type 'std::istream&' from expression of type 'int'

Question

I am trying to create a class that works with rational numbers and performing operator overloading on them. I am having issue on one part of the program, the input stream.

I am supposed to take input in the format "12/8" for example, and it should store 12 into a variable a and then 8 into a variable b.

Here is my code:

istream& operator>>( istream& In, Rational& Item )
{
    char division_sign;
    int a,b;

    In >> a >> division_sign;
    if (division_sign != '/' || !In.good())
    {
        In.setstate( ios::failbit );
    }
    else
    {
        In >> b;
        if (b != 0 || !In.good())
        {
        return Item.numerator_ = a, Item.denominator_ = b;
        }
    }
}

And here is the error I receive:

In function 'std::istream& operator>>(std::istream&, Rational&)':
131: error: invalid initialization of reference of type 'std::istream&' from expression of type 'int'

Line 131 is the return statement

Related: http://stackoverflow.com/questions/9825768/safer-but-easy-to-use-and-flexible-c-alternative-to-sscanf/9832875#9832875 — Mooing Duck, Feb 26 '13 at 18:39

score 5 · Accepted Answer · edited May 23 '17 at 11:57

5

The compilation error you observe is due to the fact that you try to return a value of another type as you've put into the declaration. You need to return In and not b:

return In;

You should therefore return the istream object reference in any possible branch of the function execution path. That is, put only one such return statement to the end of the function.

Also have a look at this FAQ on operator overloading.

edited May 23 '17 at 11:57

Community

1
1

answered Feb 26 '13 at 18:33

moooeeeep

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So now I get this error: warning: control reaches end of non-void function – Goose Feb 26 '13 at 18:41
1

@Goose: then you should have a look at my second paragraph. If you declare a function to return a certain type, you must ensure that you can always return something of that type. – moooeeeep Feb 26 '13 at 18:43
I reread your paragraphs and thought about it for a while then "That is, put only one such return statement to the end of the function." after reading that I figured out where the return statement should go. Thanks. Not sure why people downvoted the question though.. – Goose Feb 26 '13 at 18:54

score 3 · Answer 2 · answered Feb 26 '13 at 19:19

There are several things wrong with your code. First, you must return In, and only In as the last thing you do in your function; return In; should be the very last line in the function, and there should be no other return in the function. Second, you access division_sign without having verified that the input succeeded (and you shouldn't set the failbit if the the input has already failed).

Roughly speaking, something like the following should do the trick:

char separator;
int nominator;
int divisor;
in >> nominator >> separator >> divisor;
if ( in && separator != '/' ) {
    in.setstate( std::ios_base::failbit );
}
if ( in ) {
    item  = Rational( nominator, divisor );
}
return in;

You might want to check the separator before reading the divisor, so that you don't extract any more characters if it's wrong. (On the other hand, you've already extracted a complete int, so it's going to be very difficult for the client to continue anyway.)

Getting invalid initialization of reference of type 'std::istream&' from expression of type 'int'

2 Answers2