user defined exception handling - c++

Question

This is a c++ program to calculate average, and grade using 5 marks.
If marks entered are greater than 100 or less than 0, student exception should be thrown.
#include<iostream>
#include<exception>
using namespace std;
class lessex:public exception
{
public:
void what()
{
cout<<"Mark less than 0"<<endl;
}
};
class morex:public exception
{
public:
void what()
{
cout<<"Mark greater than 100"<<endl;
}
};
class student
{
string name;
string rollno;
int marks[5];
double avg;
char g;
public:
void get();
void aveg();
void grade();
void print();
};
void student::get()
{
cin>>name;
cin>>rollno;
for(int i=0;i<5;i++)
{
try{
cin>>marks[i];
if(marks[i]>100)
{
morex d;
throw d;
}
}
catch(morex &e)
{
/*e.what();*/
throw ;
}
try{
if(marks[i]<0)
{
lessex d;
throw d;
}
}
catch(lessex &e)
{
/*e.what();*/
throw ;
}
}
}
void student::aveg()
{
int sum=0;
for(int i=0;i<5;i++)
{
sum=sum+marks[i];
}
avg=sum/5;
}
void student::grade()
{
if(avg>90)
g='S';
else
g='Z';
}
void student::print()
{
cout<<name<<endl;
cout<<rollno<<endl;
cout<<g<<endl;
}
int main()
{
student s;morex e;lessex e1;
try{
s.get();
}
catch(morex &e)
{
e.what();
}
catch(lessex &e1)
{
e1.what();
}
s.aveg();
s.grade();
s.print();
return 0;
}
However, my program does not successfully exit after encountering exception in the main function.
Why is it continuing with s.aveg,grade,etc.

Answer 1

Why is my program not exiting after encountering exception-from main function? Why is it continuing with s.aveg,grade,etc.
You catch the exception, and then leave the catch block. Execution continues normally after that. Exceptions aren't re-thrown automatically at the end of a handler's block. That would maddening, what's the point of catching if you can't handle the error and continue running?
If you want the exception re-thrown, you need to add an explicit throw; in the handler. Like you already do in student::get(). Or just not have a try-catch block there. The program will terminate without "s.aveg,grade,etc." being executed.
Or, assuming you intent is not terminate, but to exit gracefully without executing other functions, you can do as user4581301 suggested. Move those function calls into the try block. That way, if an exception is thrown before their execution, they will not run before or after the handler.

Answer 2

You continue execution, without exiting, after catching the exception, and that's why the program isn't exiting.
First, you should follow the convention that what returns a string and doesn't print anything:
class lessex : public exception
{
public:
const char* what() const noexcept override
{
return "Mark less than 0";
}
};
class morex : public exception
{
public:
const char* what() const noexcept override
{
return "Mark greater than 100";
}
};
Then, you're overcomplicating things rather a lot in get;
void student::get()
{
cin >> name;
cin >> rollno;
for(int i = 0; i < 5; i++)
{
cin >> marks[i];
if (marks[i]>100)
{
throw morex();
}
if(marks[i]<0)
{
throw lessex();
}
}
}
and exceptions shouldn't be used like error codes and caught after each potentially throwing call, you normally write the "happy path" (the assumed-to-be-error-free path) and handle exceptions outside it:
int main()
{
try
{
// This part handles the normal case, assuming that all goes well.
student.s;
s.get();
s.aveg();
s.grade();
s.print();
}
// This part handles the exceptional case when something goes wrong.
catch (std::exception& ex)
{
std::cerr << ex.what();
}
}
(Your design is somewhat questionable since throwing from get can leave the object in an invalid state. You might want to rethink it.)

Answer 3

It is continuing because after you catch the exception you don't do anything about it. You have to specify what you would do inside the catch block when the specific exception is thrown.
Alternatively you could also move the other functions like s.aveg(); s.grade(); s.print(); inside try{
s.get();
}
This will prevent the aveg, grade and print functions from stop executing once an exeption is hit