Exception Handling in C++: Exception Propagation

An exception is thrown through a chain of calling functions until it is caught or it reaches to the main function.

You now know how to declare an exception and how to throw an exception. When an excep­tion is thrown, it can be caught and handled in a try-catch block, as follows:

try

{

statements; // Statements that may throw exceptions

}

catch (Exception1& exl)

{

handler for exception1;

}

catch (Exception2& ex2)

{

handler for exception2;

}

…

catch (ExceptionN& exN)

{

handler for exceptionN;

}

If no exceptions arise during the execution of the try block, the catch blocks are skipped.

If one of the statements inside the try block throws an exception, C++ skips the remaining statements in the try block and starts the process of finding the code to handle the excep­tion. This code, called the exception handler, is found by propagating the exception backward exception handler through a chain of function calls, starting from the current function. Each catch block is examined in turn, from first to last, to see whether the type of the exception object is an instance of the exception class in the catch block. If so, the exception object is assigned to the variable declared, and the code in the catch block is executed. If no handler is found, C++ exits this function, passes the exception to the function that invoked the function, and contin­ues the same process to find a handler. If no handler is found in the chain of functions being invoked, the program prints an error message on the console and terminates. The process of finding a handler is called catching an exception.

Suppose the main function invokes functionl, functionl invokes function2, function2 invokes function3, and function3 throws an exception, as shown in Figure 16.3. Consider the following scenario:

• If the exception type is Exception3, it is caught by the catch block for han­dling exception ex3 in function2. statement5 is skipped, and statement6 is executed.

• If the exception type is Exception2, function2 is aborted, the control is returned to function1, and the exception is caught by the catch block for han­dling exception ex2 in function1. statement3 is skipped, and statement4 is executed.

• If the exception type is Exception!., function!, is aborted, the control is returned to the main function, and the exception is caught by the catch block for handling exception ex1 in the main statement! is skipped, and statement2 is executed.
• If the exception is not caught in function2, functionl, and main, the program terminates. statement! and statement2 are not executed.

Source: Liang Y. Daniel (2013), Introduction to programming with C++, Pearson; 3rd edition.