This one of those typical Rational number programs. It takes a user-supplied fraction and then another. Then it does the 4 basic math operations on it. Finally it compares the fractions to each other. I have the program working fine. I just want to know how to fix this warning message. I don't quite know "what to return" from this block of code. TIA!
Wow, after all that I forgot to add the warning message:

warning: control reaches end of non-void function
//allows cin >> f; (f is a fraction)
istream &operator>>(istream &input, Rational &f)
{
    string stringy;

    //stringify the input stream
    input >> stringy;

    //find where the "/" is at in the string
    int marker = stringy.find_first_of("/");
    
    //numerator is from start of number to "/"
    string top = stringy.substr(0, marker);

    //denominator is from "/" to end
    string bottom = stringy.substr(marker + 1, stringy.size() - marker);

    //convert strings to ints
    istringstream up(top);
    int topper;
    up >> topper;
    istringstream low(bottom);
    int lower;
    low >> lower;
    
    //set fraction
    f.numer = topper;
    f.denom = lower;
}

Here is all the code, for reference. It is separated into 3 files. Header, implementation, and driver.

//Rational.h
#ifndef RATIONAL_H_
#define RATIONAL_H_

#include <iostream>
using std::istream;
using std::ostream;

class Rational
{
    friend istream &operator>>(istream &, Rational&);
    friend ostream &operator<<(ostream &, const Rational&);
public:
    Rational(int = 0, int = 1);

    Rational operator+(const Rational &);
    Rational operator-(const Rational &);
    Rational operator*(const Rational &);
    Rational operator/(const Rational &);

    bool operator==(const Rational &) const;
    bool operator!=(const Rational &) const;
    bool operator>(const Rational &) const;
    bool operator<(const Rational &) const;
    bool operator>=(const Rational &) const;
    bool operator<=(const Rational &) const;

private:
    int numer;
    int denom;
    void simplify();
};
#endif /*RATIONAL_H_*/


//Rational.cpp
#include <iostream>
using std::cout;
using std::istream;
using std::ostream;

#include <string>
using std::string;

#include <sstream>
using std::istringstream;

#include "Rational.h"

//No Denominators <= 0
Rational::Rational(int n, int d)
{
    if (d==0)
    {
        cout << "Illegal Denominator of 0, or negative Denominator\n";
        cout << "Setting Denominator to default 1\n";
        d = 1;
    }
    else if (d<0)
    {
        cout << "Denominator is Negative...\n";
        cout << "Reversing Sign of Numerator and Denominator\n";
        n = -n;
        d = -d;
    }

    numer = n;
    denom = d;
}

//allows cin >> f; (f is a fraction)
istream &operator>>(istream &input, Rational &f)
{
    string stringy;

    //stringify the input stream
    input >> stringy;

    //find where the "/" is at in the string
    int marker = stringy.find_first_of("/");
    
    //numerator is from start of number to "/"
    string top = stringy.substr(0, marker);

    //denominator is from "/" to end
    string bottom = stringy.substr(marker + 1, stringy.size() - marker);

    //convert strings to ints
    istringstream up(top);
    int topper;
    up >> topper;
    istringstream low(bottom);
    int lower;
    low >> lower;
    
    //set fraction
    f.numer = topper;
    f.denom = lower;
}

//allows cout << f; (f is a fraction)
ostream &operator<<(ostream &output, const Rational &f)
{
    return output << f.numer << "/" << f.denom;
}

//Add 2 Rationals
Rational Rational::operator+(const Rational &add)
{
    Rational temp;

    temp.numer = add.numer * denom;
    temp.numer += add.denom * numer;
    temp.denom = add.denom * denom;
    temp.simplify();
    return temp;
}

//Subtract 2 Rationals
Rational Rational::operator-(const Rational &sub)
{
    Rational temp;

    temp.numer = sub.denom * numer;
    temp.numer -= denom * sub.numer;
    temp.denom = sub.denom * denom;
    temp.simplify();
    return temp;
}

//Multiply 2 Rationals
Rational Rational::operator*(const Rational &mult)
{
    Rational temp;

    temp.numer = mult.numer * numer;
    temp.denom = mult.denom * denom;
    temp.simplify();
    return temp;
}

//Divide 2 Rationals
Rational Rational::operator/(const Rational &div)
{
    Rational temp;
    temp.numer = div.denom * numer;
    temp.denom = denom * div.numer;
    temp.simplify();
    return temp;
}

//Test Rational == Rational
bool Rational::operator==(const Rational &eq) const
{
    return (numer == eq.numer && denom == eq.denom);
}

//Test Rational != Rational
bool Rational::operator!=(const Rational &dne) const
{
    return (numer != dne.numer && denom != dne.denom);
}

//Test Rational > Rational
bool Rational::operator>(const Rational &gt) const
{
    Rational temp;
    temp.numer = numer * gt.denom;
    temp.denom = denom * gt.numer;
    return (temp.numer > temp.denom);
}

//Test Rational < Rational
bool Rational::operator<(const Rational &lt) const
{
    Rational temp;
    temp.numer = numer * lt.denom;
    temp.denom = denom * lt.numer;
    return (temp.numer < temp.denom);
}

//Test Rational >= Rational
bool Rational::operator>=(const Rational &gte) const
{
    Rational temp;
    temp.numer = numer * gte.denom;
    temp.denom = denom * gte.numer;
    return (temp.numer >= temp.denom);
}

//Test Rational <= Rational
bool Rational::operator<=(const Rational &lte) const
{
    Rational temp;
    temp.numer = numer * lte.denom;
    temp.denom = denom * lte.numer;
    return (temp.numer <= temp.denom);
}

//simplify Rational
void Rational::simplify()
{
    int largest;
    largest = numer > denom ? numer : denom;

    int gcd = 0;

    for (int loop = 2; loop<=largest; loop++)
        if (numer % loop == 0 && denom % loop == 0)
            gcd = loop;
    if (gcd != 0)
    {
        numer /= gcd;
        denom /= gcd;
    }
}

//driver.cpp
#include <iostream>
using std::cin;
using std::cout;
using std::endl;

#include <string>
using std::string;

#include <iomanip>
using std::ios_base;

#include "Rational.h"

int main()
{
    Rational a, b;

    cout << "Enter the first fraction:" << endl;
    cin >> a;

    cout << "Enter the next fraction:" << endl;
    cin >> b;

    cout << "Here are your fractions:" << endl;
    cout << a << endl;
    cout << b << endl;

    cout << "Okay, let's do some math:" << endl;
    cout << a << " + " << b << " = " << a+b << endl;
    cout << a << " - " << b << " = " << a-b << endl;
    cout << a << " * " << b << " = " << a*b << endl;
    cout << a << " / " << b << " = " << a/b << endl;

    //boolalpha used to display true or false
    cout << "Okay, let's compare them:" << endl;
    cout << a << " == " << b << " ? ";
    cout.setf(ios_base::boolalpha);
    cout << (a==b) << endl;
    cout << a << " != " << b << " ? ";
    cout.setf(ios_base::boolalpha);
    cout << (a!=b) << endl;
    cout << a << " >  " << b << " ? ";
    cout.setf(ios_base::boolalpha);
    cout << (a>b) << endl;
    cout << a << " <  " << b << " ? ";
    cout.setf(ios_base::boolalpha);
    cout << (a<b) << endl;
    cout << a << " >= " << b << " ? ";
    cout.setf(ios_base::boolalpha);
    cout << (a>=b) << endl;
    cout << a << " <= " << b << " ? ";
    cout.setf(ios_base::boolalpha);
    cout << (a<=b) << endl;
    return 0;
}

>>I don't quite know "what to return" from this block of code
It means that function must return something. You need to add return input; on line 29 or the first code you posted.

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