54. Throw statement

throw − A program throws an exception when a problem shows up. This is done using a throw keyword.

Throwing Exceptions

Exceptions can be thrown anywhere within a code block using throw statement. The operand of the throw statement determines a type for the exception and can be any expression and the type of the result of the expression determines the type of exception thrown.

Following is an example of throwing an exception when dividing by zero condition occurs −

double division(int a, int b) {
   if( b == 0 ) {
      throw "Division by zero condition!";
   }
   return (a/b);
}

you may use throw keyword when you explicitly want to throw an exception. The keyword throw is followed by a variable of a primitive data-type or an object of a type that you want to throw. Using throw keyword, an exception can be explicitly thrown from within the two places in a program –

try-block or,
catch-block.

throw in try block

To throw an exception out of try block, keyword throw is followed by a certain type of variable that we want to throw. For example –
Another way to throw a type exception, from within a try block.

double d = 10.3;
try
{
throw d;       // An exception of type int is caught by this catch block and the same type of exception is thrown.
}

throw in catch block

To throw an exception out of a catch block, keyword throw is followed by a certain type of variable that was already caught by this catch block or a different type of exception. For example –

A catch block to throw a type exception, which it has already caught.
A shortcut way to catch block to throw a type exception, which it has already caught.
A catch block to throw a different type of exception, which it has already caught.

catch(int i)
{
double d = 10.2;
throw d;       // An exception of type int is caught by this catch block but an exception of type double is thrown.
}

Using throw keyword to throw an exception out of try block

#include<iostream>

using namespace std;

int main()
{
char c = 'c';
try
{
	throw(c);   // "throw", explicitly throws an exception object of any type 
}
catch(char c)
{
	cout<<"Thrown Exception : char type ";
}
}

Output is -:

Thrown Exception : char type

As you can see in the code above, an exception object of type char is explicitly thrown using the keyword throw. This exception is then caught in the catch block which is declared with a matching exception-type, i.e. char. In the output, we have printed a message, that displays the type of exception caught in catch block.

Using throw keyword, to throw an exception caught in the catch block

#include<iostream>
using namespace std;

int main()
{
try
{
	throw(1); //throws an exception of type int
}
catch(int i)
{
	cout<<"Exception caught - int type \n";
	cout<<"Caught exception is thrown again \n";
	throw i; //throwning the caught exception of type int
}

}

Output –

Exception caught - int type
Caught exception is thrown again
terminate called after throwing an instance of 'int'

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.

In this code, an exception of type int is thrown from try block.
Next to try block, a catch block is declared with int type. Hence, the catch block catches the exception int and this exception is thrown again using the throw keyword, which leads to the ending of program abruptly.