D Programming - Exception Handling

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An exception is a problem that arises during the execution of a program. A D exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero.

Exceptions provide a way to transfer control from one part of a program to another. D exception handling is built upon three keywords try, catch, and throw.

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

  • catch − A program catches an exception with an exception handler at the place in a program where you want to handle the problem. The catch keyword indicates the catching of an exception.

  • try − A try block identifies a block of code for which particular exceptions are activated. It is followed by one or more catch blocks.

Assuming a block will raise an exception, a method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following −

try { 
   // protected code 
} 
catch( ExceptionName e1 ) { 
   // catch block 
} 
catch( ExceptionName e2 ) { 
   // catch block 
} 
catch( ExceptionName eN ) { 
   // catch block 
}

You can list down multiple catch statements to catch different type of exceptions in case your try block raises more than one exception in different situations.

Throwing Exceptions in D

Exceptions can be thrown anywhere within a code block using throw statements. The operand of the throw statements 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.

The following example throws an exception when dividing by zero condition occurs −

Example

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

Catching Exceptions in D

The catch block following the try block catches any exception. You can specify what type of exception you want to catch and this is determined by the exception declaration that appears in parentheses following the keyword catch.

try { 
   // protected code 
} 

catch( ExceptionName e ) { 
   // code to handle ExceptionName exception 
}

The above code catches an exception of ExceptionName type. If you want to specify that a catch block should handle any type of exception that is thrown in a try block, you must put an ellipsis,..., between the parentheses enclosing the exception declaration as follows −

try { 
   // protected code 
} 

catch(...) { 
   // code to handle any exception 
}

The following example throws a division by zero exception. It is caught in catch block.

Live Demo 
import std.stdio; 
import std.string;
  
string division(int a, int b) { 
   string result = "";  
   
   try {  
      if( b == 0 ) {
         throw new Exception("Cannot divide by zero!"); 
      } else { 
         result = format("%s",a/b); 
      } 
   } catch (Exception e) { 
      result = e.msg; 
   }
   
   return result; 
} 
 
void main () { 
   int x = 50; 
   int y = 0;  
   
   writeln(division(x, y));  
   
   y = 10; 
   writeln(division(x, y)); 
}

When the above code is compiled and executed, it reads the file created in previous section and produces the following result −

Cannot divide by zero!
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