Exception Header in C++ With Examples

C++ provides a list of standard exceptions defined in header <exception> in namespace std where "exception" is the base class for all standard exceptions. All exceptions like bad_alloc, bad_cast, runtime_error, etc generated by the standard library inherit from std::exception. Therefore, all standard exceptions can be caught by reference. 
Header File:

#include <exception>

Below are the errors thrown in C++:

Below is the program to illustrate some of the errors in the exception class in C++
Program 1: Below is the illustration of the std::bad_alloc error: 

// C++ program to illustrate bad_alloc
// using class bad_alloc
#include <exception>
#include <iostream>
using namespace std;

// Function to illustrate bad_alloc
void createArray(int N)
{
    // Try Block
    try {
        // Create an array of length N
        int* array = new int[N];

        // If created successfully then
        // print the message
        cout << "Array created successfully"
             << " of length " << N << " \n";
    }

    // Check if the memory
    // was allocated or not
    // using class bad_alloc
    catch (bad_alloc& e) {

        // If not, print the error message
        cout << e.what()
             << " for array of length "
             << N << " \n";
    }
}

// Driver Code
int main()
{
    // Function call to create an
    // array of 1000 size
    createArray(1000);

    // Function call to create an
    // array of 1000000000 size
    createArray(1000000000);
    return 0;
}

// C++ program to illustrate bad_alloc
// using class exception
#include <exception>
#include <iostream>
using namespace std;

// Function to illustrate bad_alloc
void createArray(int N)
{
    // Try Block
    try {
        // Create an array of length N
        int* array = new int[N];

        // If created successfully then
        // print the message
        cout << "Array created successfully"
             << " of length " << N << " \n";
    }

    // Check if the memory
    // was allocated or not
    // using class exception
    catch (exception& e) {

        // If not, print the error message
        cout << e.what()
             << " for array of length "
             << N << " \n";
    }
}

// Driver Code
int main()
{
    // Function call to create an
    // array of 1000 size
    createArray(1000);

    // Function call to create an
    // array of 1000000000 size
    createArray(1000000000);
    return 0;
}

Array created successfully of length 1000 
Array created successfully of length 1000000000 

Explanation:

• For creating an array of length 1000 the memory allocation was successful and there was no exception thrown for the same.
• For creating an array of length 1000 the memory allocation was not successful and the exception "std::bad_alloc" was thrown. The exception thrown is of type bad_alloc which is derived from the class exception. The function what() is a virtual function defined in the base class exception. The function what() returns a null-terminated string which is generally a description of an error.

Note: bad_alloc exception is thrown by operator "new" when memory allocation fails. 

Why did we catch an exception by reference? 

Catching an exception by value will call the copy constructor and create a copy of the exception which adds run-time overhead. Thus, catching by reference is a better option. If we want to modify an exception or add some additional information to the error message then catching by reference is best for it. For this case: 

catch (std::string s)
{
    s += "Additional info";
    throw;
}

The above program wants to catch the exception, add some information to it and re-throw it. But str is a call-by-value variable that gets changed locally in the function and when the function re-throws the exception then the original exception is passed. 

Correct Code: 

catch (std::string& s)
{
    s += "Additional info";
    throw;
}

Program 2: Below is the program to illustrate the logic_error: 

// C++ program to illustrate logic_error
#include <exception>
#include <iostream>
#include <stdexcept>
using namespace std;

// Function to find factorial of N and
// throws error if occurs
void findFactorial(int N)
{
    // Initialise variable by 1
    int factorial = 1;

    // Check for errors
    try {
        // If N is less than zero then,
        // it shows errors as factorial
        // of negative number can't be
        // calculated
        if (N < 0) {

            // Exception object which
            // returns the message passed
            // to it
            throw invalid_argument("negative not allowed");
        }

        // Find factorial if no error occurs
        for (int i = N; i > 0; i--) {
            factorial *= i;
        }
        cout << "Factorial of " << N << " is " << factorial
             << endl;
    }

    // Print the error message
    catch (exception& e) {
        cout << e.what();
    }
}

// Driver Code
int main()
{
    // Function call to find factorial
    // of 0
    findFactorial(0);

    // Function call to find factorial
    // of 3
    findFactorial(3);

    // Function call to find factorial
    // of -1
    findFactorial(-1);

    return 0;
}

Factorial of 0 is 1
Factorial of 3 is 6
negative not allowed

mitrukahitesh

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