- Learn C++ - https://www.learncpp.com -

14.2 — Basic exception handling

In the previous lesson on the need for exceptions [1], we talked about how using return codes causes your control flow and error flow to be intermingled, constraining both. Exceptions in C++ are implemented using three keywords that work in conjunction with each other: throw, try, and catch.

Throwing exceptions

We use signals all the time in real life to note that particular events have occurred. For example, during American football, if a player has committed a foul, the referee will throw a flag on the ground and whistle the play dead. A penalty is then assessed and executed. Once the penalty has been taken care of, play generally resumes as normal.

In C++, a throw statement is used to signal that an exception or error case has occurred (think of throwing a penalty flag). Signaling that an exception has occurred is also commonly called raising an exception.

To use a throw statement, simply use the throw keyword, followed by a value of any data type you wish to use to signal that an error has occurred. Typically, this value will be an error code, a description of the problem, or a custom exception class.

Here are some examples:

Each of these statements acts as a signal that some kind of problem that needs to be handled has occurred.

Looking for exceptions

Throwing exceptions is only one part of the exception handling process. Let’s go back to our American football analogy: once a referee has thrown a penalty flag, what happens next? The players notice that a penalty has occurred and stop play. The normal flow of the football game is disrupted.

In C++, we use the try keyword to define a block of statements (called a try block). The try block acts as an observer, looking for any exceptions that are thrown by any of the statements within the try block.

Here’s an example of a try block:

Note that the try block doesn’t define HOW we’re going to handle the exception. It merely tells the program, “Hey, if any of the statements inside this try block throws an exception, grab it!”.

Handling exceptions

Finally, the end of our American football analogy: After the penalty has been called and play has stopped, the referee assesses the penalty and executes it. In other words, the penalty must be handled before normal play can resume.

Actually handling exceptions is the job of the catch block(s). The catch keyword is used to define a block of code (called a catch block) that handles exceptions for a single data type.

Here’s an example of a catch block that will catch integer exceptions:

Try blocks and catch blocks work together -- A try block detects any exceptions that are thrown by statements within the try block, and routes them to the appropriate catch block for handling. A try block must have at least one catch block immediately following it, but may have multiple catch blocks listed in sequence.

Once an exception has been caught by the try block and routed to a catch block for handling, the exception is considered handled, and execution will resume as normal after the catch block.

Catch parameters work just like function parameters, with the parameter being available within the subsequent catch block. Exceptions of fundamental types can be caught by value, but exceptions of non-fundamental types should be caught by const reference to avoid making an unnecessary copy.

Just like with functions, if the parameter is not going to be used in the catch block, the variable name can be omitted:

This can help prevent compiler warnings about unused variables.

Putting throw, try, and catch together

Here’s a full program that uses throw, try, and multiple catch blocks:

Running the above try/catch block would produce the following result:

We caught an int exception with value -1
Continuing on our merry way

A throw statement was used to raise an exception with the value -1, which is of type int. The throw statement was then caught by the enclosing try block, and routed to the appropriate catch block that handles exceptions of type int. This catch block printed the appropriate error message.

Once the exception was handled, the program continued as normal after the catch blocks, printing “Continuing on our merry way”.

Recapping exception handling

Exception handling is actually quite simple, and the following two paragraphs cover most of what you need to remember about it:

When an exception is raised (using throw), execution of the program immediately jumps to the nearest enclosing try block (propagating up the stack if necessary to find an enclosing try block -- we’ll discuss this in more detail next lesson). If any of the catch handlers attached to the try block handle that type of exception, that handler is executed and the exception is considered handled.

If no appropriate catch handlers exist, execution of the program propagates to the next enclosing try block. If no appropriate catch handlers can be found before the end of the program, the program will fail with an exception error.

Note that the compiler will not perform implicit conversions or promotions when matching exceptions with catch blocks! For example, a char exception will not match with an int catch block. An int exception will not match a float catch block.

That’s really all there is to it. The rest of this chapter will be dedicated to showing examples of these principles at work.

Exceptions are handled immediately

Here’s a short program that demonstrates how exceptions are handled immediately:

This program is about as simple as it gets. Here’s what happens: the throw statement is the first statement that gets executed -- this causes an exception of type double to be raised. Execution immediately moves to the nearest enclosing try block, which is the only try block in this program. The catch handlers are then checked to see if any handler matches. Our exception is of type double, so we’re looking for a catch handler of type double. We have one, so it executes.

Consequently, the result of this program is as follows:

We caught a double of value: 4.5

Note that “This never prints” is never printed, because the exception caused the execution path to jump immediately to the exception handler for doubles.

A more realistic example

Let’s take a look at an example that’s not quite so academic:

In this code, the user is asked to enter a number. If they enter a positive number, the if statement does not execute, no exception is thrown, and the square root of the number is printed. Because no exception is thrown in this case, the code inside the catch block never executes. The result is something like this:

Enter a number: 9
The sqrt of 9 is 3

If the user enters a negative number, we throw an exception of type const char*. Because we’re within a try block and a matching exception handler is found, control immediately transfers to the const char* exception handler. The result is:

Enter a number: -4
Error: Can not take sqrt of negative number

By now, you should be getting the basic idea behind exceptions. In the next lesson, we’ll do quite a few more examples to show how flexible exceptions are.

What catch blocks typically do

If an exception is routed to a catch block, it is considered “handled” even if the catch block is empty. However, typically you’ll want your catch blocks to do something useful. There are three common things that catch blocks do when they catch an exception:

First, catch blocks may print an error (either to the console, or a log file).

Second, catch blocks may return a value or error code back to the caller.

Third, a catch block may throw another exception. Because the catch block is outside of the try block, the newly thrown exception in this case is not handled by the preceding try block -- it’s handled by the next enclosing try block.

14.3 -- Exceptions, functions, and stack unwinding [2]
Index [3]
14.1 -- The need for exceptions [4]