Why wont my program execute the second catch block? - c++

Question

I'm new to exception handling and this is a question about a problem I'm having on one of my first assignments on the subject. I have purposely gave the program an invalid entry for hours and seconds to test out the try/catch, so both should throw an exception. I kind of get why the first catch block catches both throws, but I'm not sure then how to get it to use both catch blocks? Thanks! (output is "ERROR: INVALID HOUR ENTRY")
int main()
{
MilTime Object;
string BadHour;
string BadSeconds;
try
{
if ( (Object.getHour() < 0) || (Object.getHour() > 2359) ) throw BadHour;
if ( (Object.getSec() < 0) || (Object.getSec() > 59 ) ) throw BadSeconds;
}
catch (string BadHour)
{
cout << "ERROR, INVALID HOUR ENTRY";
}
catch (string BadSeconds)
{
cout << "ERROR, INVALID SECOND ENTRY";
}
return 0;

Answer 1

catches are differentiated by type, not by argument names.
In principle you could define different exception types, or you could use e.g. std::system_error which allows you to pass an integer error id.
However, in practice the particular cause of an exception is of little interest other than for logging, because it generally doesn't affect what to do about it. An exception says that the code has failed to do what its contract said it should do. A catch can only try to do whatever that goal was, again (possibly in some different way), or in turn fail.

Answer 2

You have to catch exceptions by data type, not by variable name. Try something more like this instead:
#include <iostream>
#include <stdexcept>
class BadHourError : public std::runtime_error
{
public:
BadHourError() : std::runtime_error("") {}
};
class BadSecondsError : public std::runtime_error
{
public:
BadSecondsError() : std::runtime_error("") {}
};
class MilTime
{
public:
int getHour() const { return ...; }
int getSec() const { return ...; }
};
int main()
{
MilTime Object;
try
{
if ( (Object.getHour() < 0) || (Object.getHour() > 2359) ) throw BadHourError();
if ( (Object.getSec() < 0) || (Object.getSec() > 59 ) ) throw BadSecondsError();
}
catch (const BadHourError &)
{
std::cout << "ERROR, INVALID HOUR ENTRY";
}
catch (const BadSecondsError &)
{
std::cout << "ERROR, INVALID SECOND ENTRY";
}
return 0;
}
Alternatively:
#include <iostream>
#include <string>
#include <stdexcept>
class BadTimeError : public std::runtime_error
{
public:
BadTimeError(const std::string &what_arg) : std::runtime_error(what_arg) {}
};
class BadHourError : public BadTimeError
{
public:
BadHourError() : BadTimeError("INVALID HOUR ENTRY") {}
};
class BadSecondsError : public BadTimeError
{
public:
BadSecondsError() : BadTimeError("INVALID SECOND ENTRY") {}
};
class MilTime
{
public:
int getHour() const { return ...; }
int getSec() const { return ...; }
};
int main()
{
MilTime Object;
try
{
if ( (Object.getHour() < 0) || (Object.getHour() > 2359) ) throw BadHourError();
if ( (Object.getSec() < 0) || (Object.getSec() > 59 ) ) throw BadSecondsError();
}
catch (const BadTimeError &ex)
{
std::cout << "ERROR, " << ex.what();
}
return 0;
}

Answer 3

You can see how try/throw/catch is indeed type dependent if you set up your own types i.e. struct's and test as follows:
#include <iostream>
using namespace std;
struct BadHr
{
};
struct BadSec
{
};
int main()
{
int test = 2;
try
{
if (test == 1) throw BadHr();
if (test == 2) throw BadSec();
}
catch (BadHr& e)
{
std::cout << "Bad hour" << std::endl;
}
catch (BadSec& e)
{
std::cout << "Bad Sec" << std::endl;
}
}