error: no match for 'operator>>' in 'std::cin >> stopat'

Question

I'm trying to get back into C++, and this is my second program in a long while. Everything compiles just peachy, until it gets to cin >> stopat; where it returns what seems to be a fairly common error: error: no match for 'operator>>' in 'std::cin >> stopat' I've looked through a few things explaining what causes this, but nothing I actually understand (due to my relative inexperience in programming). What causes this error, and how do I fix it in case I come across it again?

#include <iostream>
#include "BigInteger.hh"

using namespace std;

int main()
{
    BigInteger A = 0;
    BigInteger  B = 1;
    BigInteger C = 1;
    BigInteger D = 1;
    BigInteger stop = 1;
    cout << "How Many steps? ";
    BigInteger stopat = 0;
    while (stop != stopat)
    {
        if (stopat == 0)
        {
            cin >> stopat;
            cout << endl << "1" << endl;
        }
        D = C;
        C = A + B;
        cout << C << endl;
        A = C;
        B = D;
        stop = stop + 1;
    }
    cin.get();
}

EDIT: Somehow, I didn't think to link the libraries referenced. Here they are: https://mattmccutchen.net/bigint/

What is `BigInteger`? Seems like it doesn't have a `>>` operator. — R. Martinho Fernandes, Feb 09 '12 at 23:32
What is BigInteger? If it is name of class then certainly there is no overloaded operator>> for it. — mikithskegg, Feb 09 '12 at 23:33

John Zwinck · Answer 1 · 2012-02-10T09:10:58.290

You haven't shown us the code for BigInteger, but there would need to be a function defined (either in BigInteger.hh or in your own code) like this:

std::istream& operator >>(std::istream&, BigInteger&);

This function would need to be implemented to actually get a "word" from a stream and try to convert it to a BigInteger. If you're lucky, BigInteger will have a constructor that takes a string, in which case it would be like this:

std::istream& operator >>(std::istream& stream, BigInteger& value)
{
    std::string word;
    if (stream >> word)
        value = BigInteger(word);
}

Edit: Now that you have pointed out the library that's being used, here's what you can do. The library itself should probably do this for you, since it provides the corresponding ostream operator, but if you look into that you will see that general-purpose, library-quality stream operators are more complex than what I'm writing here.

#include <BigIntegerUtils.hh>

std::istream& operator >>(std::istream& stream, BigInteger& value)
{
    std::string word;
    if (stream >> word)
        value = stringToBigInteger(word);
}

`value = BigInteger(word)` might involve an unnecessary copy, since the `BigInteger` class surely has a member function to parse the number from a string (atleast it would make sense if it provides such a constructor). — Xeo, Feb 10 '12 at 06:42
Since I wrote my answer the OP posted a link to the actual library in question, so now we can know for sure. I'll update my answer accordingly. — John Zwinck, Feb 10 '12 at 09:06

score 0 · Answer 2 · answered Feb 09 '12 at 23:39

What you've left out here is details about your BigInteger class. In order to read one from an input stream with the >> operator, you need to define operator>> (often called a stream extractor) for your class. That's what the compiler error you're getting means.

Essentially, what you need is a function that looks like this:

std::istream &operator>>(std::istream &is, BigInteger &bigint)
{ 
    // parse your bigint representation there
    return is;
}

error: no match for 'operator>>' in 'std::cin >> stopat'

2 Answers2

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