linker error

[snickerst]

When using the interval_since member function I get a:
[Linker error] undefined reference to `DigitalTime::interval_since(DigitalTime const&, int&, int&) const' and I'm not sure why. All the files I'm using are below:


///Header file dtime.h:
#include <iostream>
using namespace std;

class DigitalTime
{
public:

void interval_since(const DigitalTime& previous,int& hours,int& mins)const;
//copied from textbook
//should return time change

friend bool operator ==(const DigitalTime& time1, const DigitalTime& time2);
//Returns true if time1 and time2 represent the same time;
//otherwise, returns false.

DigitalTime(int the_hour, int the_minute);
//Precondition: 0 <= the_hour <= 23 and 0 <= the_minute <= 59.
//Initializes the time value to the_hour and the_minute.

DigitalTime( );
//Initializes the time value to 0:00 (which is midnight).

void advance(int minutes_added);
//Precondition: The object has a time value.
//Postcondition: The time has been changed to minutes_added minutes later.

void advance(int hours_added, int minutes_added);
//Precondition: The object has a time value.
//Postcondition: The time value has been advanced
//hours_added hours plus minutes_added minutes.

friend istream& operator >>(istream& ins, DigitalTime& the_object);
//Overloads the >> operator for input values of type DigitalTime.
//Precondition: If ins is a file input stream, then ins has already been
//connected to a file.

friend ostream& operator <<(ostream& outs, const DigitalTime& the_object);
//Overloads the << operator for output values of type DigitalTime.
//Precondition: If outs is a file output stream, then outs has already been
//connected to a file.


private:
int hour;
int minute;
};
//-------------------------------------------------------------------

//DISPLAY 12.2 Implementation File for DigitalTime
//Implementation file dtime.cpp
//This is the IMPLEMENTATION of the ADT DigitalTime.
//The interface for the class DigitalTime is in the header file dtime.h.
#include <iostream>
#include <cctype>
#include <cstdlib>
#include "dtime.h"
using namespace std;

//These FUNCTION DECLARATIONS are for use in the definition of
//the overloaded input operator >>:

void DigitalTime::interval_since(const DigitalTime& previous,int& hours,int& mins);


void read_hour(istream& ins, int& the_hour);
//Precondition: Next input in the stream ins is a time in 24-hour notation,
//like 9:45 or 14:45.
//Postcondition: the_hour has been set to the hour part of the time.
//The colon has been discarded and the next input to be read is the minute.

void read_minute(istream& ins, int& the_minute);
//Reads the minute from the stream ins after read_hour has read the hour.

int digit_to_int(char c);
//Precondition: c is one of the digits '0' through '9'.
//Returns the integer for the digit; for example, digit_to_int('3') returns 3.

bool operator ==(const DigitalTime& time1, const DigitalTime& time2)
{
return (time1.hour == time2.hour && time1.minute == time2.minute);
}

//Uses iostream and cstdlib:
DigitalTime::DigitalTime(int the_hour, int the_minute)
{
if (the_hour < 0 || the_hour > 23 || the_minute < 0 || the_minute > 59)
{
cout << "Illegal argument to DigitalTime constructor.";
exit(1);
}

else
{
hour = the_hour;
minute = the_minute;
}
}
DigitalTime::DigitalTime( ) : hour(0), minute(0)
{
//Body intentionally empty.
}


void DigitalTime::advance(int minutes_added)
{
int gross_minutes = minute + minutes_added;
minute = gross_minutes%60;

int hour_adjustment = gross_minutes/60;
hour = (hour + hour_adjustment)%24;
}

void DigitalTime::advance(int hours_added, int minutes_added)
{
hour = (hour + hours_added)%24;
advance(minutes_added);
}

//Uses iostream:
ostream& operator <<(ostream& outs, const DigitalTime& the_object)
{
outs << the_object.hour << ':';
if (the_object.minute < 10)
outs << '0';
outs << the_object.minute;
return outs;
}

//Uses iostream:
istream& operator >>(istream& ins, DigitalTime& the_object)
{
read_hour(ins, the_object.hour);
read_minute(ins, the_object.minute);
return ins;
}

int digit_to_int(char c)
{
return ( static_cast<int>(c) - static_cast<int>('0') );
}

//Uses iostream, cctype, and cstdlib:
void read_minute(istream& ins, int& the_minute)
{
char c1, c2;
ins >> c1 >> c2;

if (!(isdigit(c1) && isdigit(c2)))


{
cout << "Error illegal input to read_minute\n";
exit(1);
}

the_minute = digit_to_int(c1)*10 + digit_to_int(c2);

if (the_minute < 0 || the_minute > 59)
{
cout << "Error illegal input to read_minute\n";
exit(1);
}
}

//Uses iostream, cctype, and cstdlib:
void read_hour(istream& ins, int& the_hour)
{
char c1, c2;
ins >> c1 >> c2;
if ( !( isdigit(c1) && (isdigit(c2) || c2 == ':' ) ) )
{
cout << "Error illegal input to read_hour\n";
exit(1);
}

if (isdigit(c1) && c2 == ':')
{
the_hour = digit_to_int(c1);
}
else //(isdigit(c1) && isdigit(c2))
{
the_hour = digit_to_int(c1)*10 + digit_to_int(c2);
ins >> c2;//discard ':'
if (c2 != ':')
{
cout << "Error illegal input to read_hour\n";
exit(1);
}
}
if ( the_hour < 0 || the_hour > 23 )
{
cout << "Error illegal input to read_hour\n";
exit(1);
}
}


void DigitalTime::interval_since(const DigitalTime& previous,int& hours,int& mins)
{
cout<<"inside interval";
}

//---------------------------------------------------------------------------


// application file for digital time

#include <cstdlib>
#include <iostream>
#include "dtime.h"

using namespace std;

int main()
{
DigitalTime time1,time2(13,50),time3;
cout<<"this program will test the DigitalTime class\n";
cout<<"Enter a time in the format(hr:min)time3: ";
cin>>time3;
cout<<"all of the following will use the overloaded <<\n";
cout<<"constructor DigitalTime()time1="<<time1;
cout<<"\nconstructor DigitalTime(int,int)time2="<<time2;
cout<<"\noverloaded DigitalTime >> time3="<<time3;
if (time2==time3)
{cout<<"\ntime2 = time3";}
else
{cout<<"\ntime2 != time3";}
cout<<"\ntesting the advance(min)";
int mintoadd;
cout<<"\nhow many min's to add?";
cin>>mintoadd;
cout<<"test3 was "<<time3;
time3.advance(mintoadd);
cout<<" now its "<<time3;
cout<<"\ntesting the advance(hour,min)";
int hourstoadd;
cout<<"\nhow many hours to add?";
cin>>hourstoadd;
cout<<"how many min's to add?";
cin>>mintoadd;
cout<<"test3 was "<<time3;
time3.advance(hourstoadd,mintoadd);
cout<<" now its "<<time3;
cout<<"\ntesting interval() now";
int hours,mins;
DigitalTime previous(2,20);
time3.interval_since(previous,hours,mins);

/* things to do
get interval_since member function to work
*/
system("PAUSE");
return EXIT_SUCCESS;
}

[vlad from moscow]

The function

void interval_since(const DigitalTime& previous,int& hours,int& mins)const;

is declared with the const qualifier. So it shall be defined with the same qualifier.