C++ Exception Handling

An exception is an unexpected event that occurs during program execution. For example,

divide_by_zero = 7 / 0;

The above code causes an exception as it is not possible to divide a number by 0.

The process of handling these types of errors in C++ is known as exception handling.

In C++, we handle exceptions with the help of the try and catch blocks, along with the throw keyword.

• try - code that may raise an exception
• throw - throws an exception when an error is detected
• catch - code that handles the exception thrown by the throw keyword

Note: The throw statement is not compulsory, especially if we use standard C++ exceptions.

Syntax for Exception Handling in C++

The basic syntax for exception handling in C++ is given below:

try {

    // code that may raise an exception
    throw argument;
}

catch (exception) {
    // code to handle exception
} 

Here, we have placed the code that might generate an exception inside the try block. Every try block is followed by the catch block.

When an exception occurs, the throw statement throws an exception, which is caught by the catch block.

The catch block cannot be used without the try block.

Example 1: C++ Exception Handling

// program to divide two numbers
// throws an exception when the divisor is 0

#include <iostream>
using namespace std;

int main() {

    double numerator, denominator, divide;

    cout << "Enter numerator: ";
    cin >> numerator;

    cout << "Enter denominator: ";
    cin >> denominator;

    try {

        // throw an exception if denominator is 0
        if (denominator == 0)
            throw 0;

        // not executed if denominator is 0
        divide = numerator / denominator;
        cout << numerator << " / " << denominator << " = " << divide << endl;
    } 

    catch (int num_exception) {
        cout << "Error: Cannot divide by " << num_exception << endl;
    }

    return 0;
}

Output 1

Enter numerator: 72
Enter denominator: 0
Error: Cannot divide by 0

Output 2

Enter numerator: 72
Enter denominator: 3
72 / 3 = 24

The above program divides two numbers and displays the result. But an exception occurs if the denominator is 0.

To handle the exception, we have put the code divide = numerator / denominator; inside the try block. Now, when an exception occurs, the rest of the code inside the try block is skipped.

The catch block catches the thrown exception and executes the statements inside it.

If none of the statements in the try block generates an exception, the catch block is skipped.

Notice that we have thrown the int literal 0 with the code throw 0;.

We can throw any literal or variable or class, depending on the situation and depending on what we want to execute inside the catch block.

The catch parameter int num_exception takes the value passed by the throw statement i.e. the literal 0.

Catching All Types of Exceptions

In exception handling, it is important that we know the types of exceptions that can occur due to the code in our try statement.

This is so that we can use the appropriate catch parameters. Otherwise, the try...catch statements might not work properly.

If we do not know the types of exceptions that can occur in our try block, then we can use the ellipsis symbol ... as our catch parameter.

try {
    // code
}
catch (...) {
    // code
} 

C++ Multiple catch Statements

In C++, we can use multiple catch statements for different kinds of exceptions that can result from a single block of code.

try {
    // code
} 
catch (exception1) {
    // code
} 
catch (exception2) {
    // code
} 
catch (...) {
    // code
}

Here, our program catches exception1 if that exception occurs. If not, it will catch exception2 if it occurs.

If there is an error that is neither exception1 nor exception2, then the code inside of catch (...) {} is executed.

Example 2: C++ Multiple catch Statements

This program divides two numbers and stores the result in an array element. There are two possible exceptions that can occur in this program:

• If the array is out of bounds i.e. if the index of the array is greater than the size of the array
• If a number is divided by 0

These exceptions are caught in multiple catch statements.

#include <iostream>
using namespace std;

int main() {
    
    double numerator, denominator, arr[4] = {0.0, 0.0, 0.0, 0.0};
    int index;
    
    cout << "Enter array index: ";
    cin >> index;
    
    try {
        
        // throw exception if array out of bounds
        if (index >= 4)
            throw "Error: Array out of bounds!";
            
        // not executed if array is out of bounds
        cout << "Enter numerator: ";
        cin >> numerator;
    
        cout << "Enter denominator: ";
        cin >> denominator;

        // throw exception if denominator is 0
        if (denominator == 0)
            throw 0;

        // not executed if denominator is 0
        arr[index] = numerator / denominator;
        cout << arr[index] << endl;
    }

    // catch "Array out of bounds" exception
    catch (const char* msg) {
        cout << msg << endl;
    }

    // catch "Divide by 0" exception
    catch (int num) {
        cout << "Error: Cannot divide by " << num << endl;
    }

    // catch any other exception
    catch (...) {
        cout << "Unexpected exception!" << endl;
    }
    
    return 0;    
}

Output 1

Enter array index: 5
Error: Array out of bounds!

Here, the array arr only has 4 elements. So, index cannot be greater than 3.

In this case, index is 5. So we throw a string literal "Error: Array out of bounds!". This exception is caught by the first catch block.

Notice the catch parameter const char* msg. This indicates that the catch statement takes a string literal as an argument.

Output 2

Enter array index: 2
Enter numerator: 5
Enter denominator: 0
Error: Cannot divide by 0

Here, the denominator is 0. So we throw the int literal 0. This exception is caught by the second catch block.

If any other exception occurs, it is caught by the default catch block.

Output 3

Enter array index: 2
Enter numerator: 5
Enter denominator: 2
2.5

Here, the program runs without any problem as no exception occurs.

C++ Standard Exception

C++ has provided us with a number of standard exceptions that we can use in our exception handling. Some of them are shown in the table below.

There are many other standard exceptions in C++.

These exceptions are defined in the exception header file.

To learn more, visit our C++ Standard Exceptions tutorial.