CMPT 212 Object-Oriented Applications Design in C++ Spring 1998

Read section Exceptions of chapter 14 from C++ Primer Plus (2nd ed.), by Stephen Prata.

Exceptions

• Exceptions are a language feature that enables programs to handle errors and other abnormal situations in an elegant way.
  • Exceptions allow the programmer to separate the error-checking and error-handling code from the "regular" code.

• The exception mechanism in C++ is implemented by the keywords try, throw, and catch.
  • The try keyword indicates a block of code in which we expect exceptions to be generated.

  • The throw keyword indicates that we are generating an exceptions.

  • The catch keyword follws a try block and indicates what we should do if an exception is generated.

• Example:

     int i;
     ...
     try {
        if (i < 0) {
  	 throw "negative number";
  	 }
        else {
  	 cout << i << endl;
  	 }
        }
     catch (char * str) {
        cout << str << endl;
        }
  	 

• When we throw an exception, we specify the value of the exception.

• Each catch clause has a type associated with it. It will catch an exception of this type.
  • If an exception is thrown which does not match the type in any of our catch clauses, then the exception will be re-thrown so that it can be caught by an exception handler higher up on the call stack.

  • The clause catch (...) will catch any exception.

• Example:

     float square_root(float f) {
        if (f < 0) {
  	 throw "negative number";
  	 }
        else {
  	 return sqrt(f);
  	 }
     }
  
     void func() {
        float f;
        cin >> f;
        try {
  	 cout << square_root(f) << endl;
  	 }
        catch (char * str) {
  	 cout << str << endl;
  	 }
        catch (int i) {
  	 cout << i << endl;
  	 }
        catch (...) {
  	 cout << "unknown exception" << endl;
  	 }
     }
  	 

• When an exception is thrown, control passes to the nearest catch clause which catches an exception of this type.
  • If the catch clause is in some enclosing function, then the inner functions are exited.

  • If there is no such catch clause, the program is halted with an "uncaught exception" run-time error.

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