C++ Exception Handling and file handling

 

C++ Exceptions

When executing C++ code, different errors can occur: coding errors made by the programmer, errors due to wrong input, or other unforeseeable things.

 

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).

 

C++ try and catch

Exception handling in C++ consist of three keywords: try, throw and catch:

 

The try statement allows you to define a block of code to be tested for errors while it is being executed.

 

The throw keyword throws an exception when a problem is detected, which lets us create a custom error.

 

The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

 

The try and catch keywords come in pairs:

 

 

try {

  // Block of code to try

  throw exception; // Throw an exception when a problem arise

}

catch () {

  // Block of code to handle errors

}

 

 

#include <iostream>

using namespace std;

 

int main() {

  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; 

  }

  return 0;

}

 

 

C++ Files

 

The fstream library allows us to work with files.

 

To use the fstream library, include both the standard <iostream> AND the <fstream> header file:

 

There are three classes included in the fstream library, which are used to create, write or read files:

 

Class       Description

 

ofstream          Creates and writes to files

ifstream           Reads from files

fstream              A combination of ofstream and ifstream: creates, reads, and writes to files

 

To create a file, use either the ofstream or fstream class, and specify the name of the file.

 

To write to the file, use the insertion operator (<<).

 

#include <iostream>

#include <fstream>

using namespace std;

 

int main() {

  ofstream MyFile("filename.txt");

 

  MyFile << "Files can be tricky, but it is fun enough!";

 

  MyFile.close();

}

 

 

Read a File

 

To read from a file, use either the ifstream or fstream class, and the name of the file.

 

Note that we also use a while loop together with the getline() function (which belongs to the ifstream class) to read the file line by line,

and to print the content of the file:

 

 

 

#include <iostream>

#include <fstream>

#include <string>

using namespace std;

 

int main () {

  ofstream MyWriteFile("filename.txt");

 

  MyWriteFile << "Files can be tricky, but it is fun enough!";

 

  MyWriteFile.close();

 

  string myText;

 

  ifstream MyReadFile("filename.txt");

 

  while (getline (MyReadFile, myText)) {

    cout << myText;

  }

 

  MyReadFile.close();

}