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In our last article, we explained what exactly happens when an exception occurs in a program, how it propagates through a program and eventually ending its execution abruptly. In this article, we are going to explain how to handle an exception thrown in a program, using the try-catch blocks and a throw keyword, so that the program does not have an abrupt end.

A try-catch block is made up of two individual blocks - a try block and a catch block, let's read about these blocks and what are they used for.

try block


In the try block we enclose a portion of program in which an exception may occur during its runtime. If an exception occurs in the portion of program enclosed within try block, it is thrown to the catch block.






catch block


The catch block declares a type of exception that it is going to catch and handle. If the declared type of exception in catch block is similar to the type of exception thrown by try block, the exception is caught within this catch block and the program continues its normal execution without ending abruptly.













throw keyword

Using throw keyword, an exception can be explicitly thrown from within the two places in a program -

• try-block or,
• catch-block.

As you saw in the last article explaining exception propagation, an uncaught/unhandled exception terminated the program's execution abruptly. In order to not let it happen, we can try to anticipate an occurence of an exception within a program in advance and can use throw keyword to explicitly throw an exception from witin a try block, so that a catch block could catch it and program does not terminate unexpectedly.

How to catch an exception using try-catch block?

Exception thrown has a certain type. In order to catch an exception of a type, we have to make sure that our catch block has declared a type of exception, similar to the type of exception that is thrown by the try block, using throw keyword.


Let's understand this by an example -:

#include<iostream>

using namespace std;

int main()
{
int a=10, b=0, c;
try
{
	//if a is divided by b(which has a value 0);
	if(b==0)
		throw(c); 
	else
	c=a/b; 		
		
}
catch(int a) //matching int type of exception is declared in "catch block"
{
	cout<<"An exception caught : division by zero \n"; //message related to caught exception is printed
}		

cout<<"Outside try-catch block";
return 0;
} 		

Output :

An exception caught : division by zero
Outside try-catch block

As you can see in the program, it finds a division-by-zero problem and an exception of type int is thrown when a statement enclosed within the try block is executed. This exception is caught by the catch block because it is declared with a matching exception of type int.

As the thrown exception of type int is caught by the catch block, hence, the program does not end abruptly and it continues to successfully execute the statement next to the try-catch block, which prints a message - "Outside try-catch block".

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Another example of using try-catch block

Let us see another example in which we are going to catch an out-of-range index exception, which gets thrown when a statement is trying to access an index of array which is out of its range. Let us see how to deal with such exception using try-catch block.

Let's understand this by an example -:

#include<iostream>
#include<string>

using namespace std;

int main() 
{
char arr[5];
try
{
	for (int n=0; n<=6; n++)
    	{
     		 if (n>4) 
			throw std::string("out-of-range");
     		 else 
			arr[n]='a';
    	}
}
catch (std::string st)
{
	std::cout << "Exception caught : " << st;
}
return 0;
}

Output :

Exception caught : out-of-range

As you can see in the program, the compiler finds one of the statements enclosed within the try block is trying to access an invalid-index or out-of-range index hence, an exception of type std:string is thrown This exception is caught by the catch block because it is declared with a matching exception of type std:string.

Mismatch in exception declared and exception thrown

• If a catch block declares a type of exception that does not match with the type of exception thrown by the try block then the exception remains uncaught and this catch block is not executed.


• Next, this uncaught exception is thrown down to the next method in the call stack. If a matching catch block is found in the next function, exception is caught, otherwise, the exception is again thrown down to the next function.


• The process continues until the thrown exception reaches the main() function(where it all began). If there is no matching catch block in the main() function, the call stack explodes and program terminates abruptly.

Let us understand this by an example here.

#include<iostream>

using namespace std;

class TryCatch2
{
public: 
void meth();
};



void TryCatch2 :: meth()
{
try
{
	int a = 10;
	int b = 10;

	int result = a-b;
	if(result==0)
	 	throw(b);
	else
	cout<<(100/result);
}
catch(double c) //Non-matching exception-type is declared in this catch block
{
	cout<<"Caught exception type : double type \n";
}
cout<<"Outside try-catch block";
} //meth1() function ends




int main()
{
TryCatch2 ob;
try
{
	ob.meth(); 	//enclosing the call to meth1() function in try block
}
catch(int c)
{
	cout<<"Caught exception : int type \n"	;
}

} //main function ends

Output:

Caught exception : int type

• Here, an exception of type int is thrown out of try block in meth() function. This thrown exception is not caught by the catch block defined in the meth() function, because the catch block has declared a non-matching type of exception i.e. double.


• This uncaught exception is thrown down to the next function in call stack, i.e. main() function, which called meth() function from within its try-catch block and it does have a matching catch block to catch the exception thrown by meth() function of type int.
  
  
  

  Note :

  
  The cout statement in meth() didn't execute because the control had left the meth() function and reached main() function when exception was thrown down the call stack.








Points to remember when declaring a try-catch block




• There should be no statement between a try-block" and its associated catch block. A catch-block should begin right after the end of try-block, otherwise there will a compile error.



#include<iostream>

using namespace std;

int main()
{
try
{
int a = 10;
	int b = 10;

	int result = a-b;
	if(result==0)
	 	throw(b);
	else
	cout<<(100/result);
}
//cout<<"Statement between try-catch"; //Statement between try-catch block leads to compile error
catch(int e)
{
	cout<<"Caught exception : int type \n"	;
}
} 





On compilation -




c142.cpp: In function 'int main()':
A.cpp:18:1: error: expected 'catch' before 'cout'
 cout<<"Statement between try-catch"; //Statement between try-catch block leads
to compile error
 ^~~~
A.cpp:18:1: error: expected '(' before 'cout'
A.cpp:18:1: error: 'cout' does not name a type
A.cpp:18:5: error: expected ')' before '<<' token
 cout<<"Statement between try-catch"; //Statement between try-catch block leads
to compile error
     ^~
A.cpp:18:5: error: expected '{' before '<<' token
A.cpp:18:5: error: expected primary-expression before '<<' token
A.cpp:19:1: error: expected primary-expression before 'catch'
 catch(int e)
 ^~~~~






• We cannot declare try block alone. It has to be declared with a catch block, otherwise there will be a compile error.



#include<iostream>

using namespace std;

int main()
{
try
{
	int a = 10;
	int b = 10;

	int result = a-b;
	if(result==0)
	 	throw(b);
	else
	cout<<(100/a-b);
}
}





On compilation -




c143.cpp: In function 'int main()':
c143.cpp:19:1: error: expected 'catch' before '}' token
 }
 ^
c143.cpp:19:1: error: expected '(' before '}' token
c143.cpp:19:1: error: expected type-specifier before '}' token
c143.cpp:19:1: error: expected ')' before '}' token
c143.cpp:19:1: error: expected '{' before '}' token


In the next article, we will talk about multiple catch blocks that we could associated with a single try block.





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