Welcome to Programming and Data Structures

Exceptions are a programming convention that allows a low-level function to communicate “there was an error” to higher-level code that gets around some of the complications of other strategies..

When a function generates an exception, we say that it throws that exception. The thrown exception travels back up the call stack one function at a time, giving each higher-level function a chance to catch the exception.. If nothing catches the exception, the program will be stopped.

To start exploring exceptions, we will use a familiar string function. When substr is given a bad index it responds by throwing an exception. Try running this sample that ends up asking for a substring starting at index 50 in a much shorter string:

If you run that code, you should get an error like:

terminate called after throwing an instance of 'std::out_of_range'

An out_of_range exception was thrown by the attempt on line 8 to get the substring. Because the exception was not caught, the program was terminated.

Let’s try catching the exception in mediumJob. To catch an exception, we use this syntax:

try {
  // code that might throw an exception
} catch (const exception& e) {
  // code to handle the exception
}

exception is a data type defined in the <exception> library. catch(const exception& e) says that the variable e is going to be a const reference to an exception.

Let’s add that to our program to catch the exception that is coming from substr:

This time, we caught the exception. The exception happens on line 11. As soon as it happens, execution jumps to the catch. Line 12 never runs. Which is good, as there is no result from line 11 that it can use!

Inside the catch, line 14 runs and prints a message. e.what() results in a string that describes the exception. The code then recovers from the error by setting the mediumResult variable to a default value ("mediumJob(?)").

That result is returned to bigJob in the normal manner and the program continues on as if nothing bad happened.

try {
---
try ( #paired
---
 foo();
---
} catch(const exception& e) {
---
} catch() { #paired
---
} catch(e) { #paired
---
 cout << "There was an issue";
---
}