Exceptions
As mentioned in the C++ Errors chapter, different types of errors can occur while running a program - such as coding mistakes, invalid input, or unexpected situations.
When an error occurs, C++ will normally stop and generate an error message. The technical term for this is: C++ will throw an exception (throw an error).
Exception Handling (try and catch)
Exception handling lets you catch and handle errors during runtime - so your program doesn't crash.
It uses three keywords
- try - defines the code to test
- throw - triggers an exception
- catch - handles the error
Example
try {
// Code that may throw an exception throw 505;
}
catch (int errorCode) {
cout << "Error occurred: " << errorCode;
}Here, the program throws an exception with the value 505 , which is caught and handled in the catch block.
Real-Life Example: Age Check
We can use exception handling to check if a user is old enough:
Example
try {
int age = 15;
if (age >= 18) {
cout << "Access granted - you are old enough.";
} else {
throw (age);
}
}
catch (int
myNum) {
cout << "Access denied - You must be at least 18 years old.\n";
cout << "Age is: " << myNum;
}We use the try block to test some code: If the age variable is less than 18 , we will throw an exception, and handle it in our catch block.
In the catch block, we catch the error and do something about it. The catch statement takes a parameter : in our example we use an int variable ( myNum ) (because we are throwing an exception of int type in the try block ( age )), to output the value of age .
If no error occurs (e.g. if age is 20 instead of 15 , meaning it will be be greater than 18), the catch block is skipped:
int age = 20;You can also use the throw keyword to output a reference number, like a custom error number/code for organizing purposes ( 505 in our example):
Example
try {
int age = 15;
if (age >= 18) {
cout << "Access granted - you are old enough.";
} else {
throw 505;
}
}
catch (int myNum) {
cout << "Access denied - You must be at least 18 years old.\n";
cout << "Error number: " << myNum;
}Handle Any Type of Exceptions (...)
If you do not know the throw type used in the try block, you can use the "three dots" syntax ( ... ) inside the catch block, which will handle any type of exception:
Example
try {
int age = 15;
if (age >= 18) {
cout << "Access granted - you are old enough.";
} else {
throw 505;
}
}
catch (...) {
cout << "Access denied - You must be at least 18 years old.\n";
}