Exception Handling Basics in C++

In C++, Exception Handling is a process to handle runtime errors. Exception is an event which is thrown at runtime in C++. All exceptions are derived from std::exception class. It is a runtime error which can be handled. It prints exception message and terminates the program, if we don't handle the exception.

Exceptions are defined in C++ standard as <exception> class that we can use inside our programs. The arrangement of parent-child class hierarchy has been shown below:

Common exception classes in C++ are:

Exception	Description
std::exception	This is an exception and parent class of all the standard C++ exceptions.
std::bad_cast	It is an exception thrown by dynamic_cast.
std::bad_exception	This exception is used to handle the unexpected exceptions in a C++ program.
std::bad_alloc	It is generally be thrown by new.
std::logic_failure	This exception can be detected by reading a code.
std::runtime_error	This exception cannot be detected by reading a code
std::bad_typeid	It is an exception thrown by typeid.

Keywords:

There are 3 keywords in exception handling: try, catch and throw.

Try/Catch block:

In C++, exception handling is performed using try/catch statement. The code that may occur exception is used to place by Try block. Catch block is used to handle the exception.

Example Code

#include <iostream>
using namespace std;
class Sample1 {
   public:
   Sample1()
   {
      cout << "Construct an Object of sample1" << endl;
   }
   ~Sample1()
   {
      cout << "Destruct an Object of sample1" << endl;
   }
};
class Sample2 {
   public:
   Sample2()
   {
      int i=7;
      cout << "Construct an Object of sample2" << endl;
      throw i;
   }
   ~Sample2()
   {
      cout << "Destruct an Object of sample2" << endl;
   }
};
int main() {
   try {
      Sample1 s1;
      Sample2 s2;
   } catch(int i) {
      cout << "Caught " << i << endl;
   }
}

Output

Construct an Object of sample1
Construct an Object of sample2
Destruct an Object of sample1
Caught 7

User Defined Exception:

We can define our own exceptions by inheriting and overriding exception class functionalities.

Example Code

#include <iostream>
#include <exception>
using namespace std;
struct DivideByZero : public exception {
   const char * what () const throw () {
      return "My Exception";
   }
};
int main() {
   try {
      throw DivideByZero();
   } catch(DivideByZero& e) {
      cout << "Exception caught" << endl;
      cout << e.what() << endl;
   } catch(exception& e) {
   }
}

Output

Exception caught
My Exception
what() = A public method provided by exception class and it has been
overridden by all the child exception classes. It returns the cause of an
exception.