CPP Errors and Exceptions

Handling Errors and Exceptions?

• The basic idea behind exceptions is fairly straightforward:
  • The computer tries to run a piece of code. This code might try to allocate resources such as memory, might try to lock a file, or any of a variety of tasks.
  • Logic (code) is included to be prepared in case the code you are trying to execute fails for some exceptional reason.
  • In case your code is being used by other code (for instance, one function calling another), you also need a mechanism to pass information about any problems (exceptions) from your level, up to the next.

  try {
     SomeDangerousFunction();
  }
  catch(OutOfMemory){
     // take some actions
  }
  catch(FileNotFound) {
     // take other action
  }
  catch (...) {              // this refer to any other exception
  }

• You can create your own exceptions, and gain the ability to have customized handlers (catch blocks) for exceptions that are meaningful to your application.
• To create an exception that causes the try statement to react, the keyword, throw, is used.

  throw exception;

• The value that you throw in the exception can be of virtually any type.
  Example that throw a character string:

  #include <iostream>
  
  using namespace std;
  
  int main() {
    int speed = 90;  // speed : meter/second
    int time = -10;    // second
  
    try   {
      cout << "Speed multiply by time (meter) = ";
      if ( time < 0 ) {
        cout << "(" << speed << "*" << time <<")?" << endl;
        throw "Time must be creater or equal to 0!";
      }
      else
        cout << (speed *time) << endl;
    }
    // as character string is thrown the try 
    // will use this catch 
    catch( const char * ex ) {
      cout << "\nError: " << ex  << endl;
    }
    catch(...) {
      cout << "Something else has gone wrong!" << endl;
    }
    return 0;
  }

  When we run this application, the result will be:

  Speed multiply by time (meter) = (90*-10)?
  
  Error: Time must be creater or equal to 0!

  You can download this example here (needed tools can be found in the right menu on this page).

• You can create much more complex classes for throwing an exception.
• You can use all you have learned to build a such class, including inheritance and Polymorphism with virtual methods.
  Example that throw class exception:

  #include <iostream>
  #include <sstream>
  #include <string>
  using namespace std;
  // Define the exception classes
  
  class Exception {
  protected:
    string * errors;
    int errorSize;
  
  public:
    Exception() :errorSize(0) {
      errors = new string[10];
      errors[errorSize] ="You got runtime error(s):\n";
      errorSize++;
    }
    virtual ~Exception(){
      delete [] errors;
    }
    void showErrors()  {
      for (int i=0 ; i< errorSize ; i++ ) {
        cout << errors[i];
      }
    }
  
  };
  
  class VectorException : public Exception {
  protected:
    int size;
  public:
    VectorException(int size):size(size) {}
    VectorException (const VectorException & refObject){
      size=refObject.size;
      for (int i=0 ; i< refObject.errorSize ; i++ ) {
        errors[i]=refObject.errors[i];
      }
      errorSize=refObject.errorSize;
    }
    void ValidateInitialSize() {
      errorSize=1;
      if (size == 0) {
        ostringstream oss;
        oss << "The vector cannot be : " << size << "\n";
        errors[errorSize]=oss.str();
        errorSize++;
      }
      if (size > 5000) {
        ostringstream oss;
        oss << "The vector is too large : " << size << "\n";
        errors[errorSize]=oss.str();
        errorSize++;
      }
      if (size < 0) {
        ostringstream oss;
        oss << "The vector cannot be less than zero : " << size << "\n";
        errors[errorSize]=oss.str();
        errorSize++;
      }
      if (size <=20) {
        ostringstream oss;
        oss << "The vector must be greater then 20 : " << size << "\n";
        errors[errorSize]=oss.str();
        errorSize++;
      }
      if (errorSize>1) {
        throw VectorException(*this);
      }
    }
  
    void ValidateBoundary(int offSet) {
      errorSize=1;
      if (offSet < 0 || offSet >= size) {
        ostringstream oss;
        oss << "The index is out of Boundary : " << offSet << "\n";
        errors[errorSize]=oss.str();
        errorSize++;
        throw VectorException(*this);
      }
    }
  };
  
  class Vector : private VectorException {
  public:
    // Constructors
    Vector(int size = 5);
    ~Vector() { delete [] pType;}
  
    // overloaded operators
    Vector& operator=(const Vector&);
    int& operator[](int offSet);
    // Accessors
    int getSize() const { return size; }
  private:
    int *pType;
    int  size;
  };
  
  Vector::Vector(int size): size(size), VectorException(size) {
    ValidateInitialSize();
    pType = new int[size];
    for (int i = 0; i < size; i++)
      pType[i] = 0;
  }
  
  int & Vector::operator [] (int offSet) {
    ValidateBoundary(offSet);
    return pType[offSet];
  }
  
  
  int main() {
  
    cout << "\nTry a vector size  > 20 but less the 100\n";
    try  {
      Vector intVector(30);
      for (int j = 0; j < 100; j++) {
        intVector[j] = j;
        cout << "intVector[" << j << "] okay..." << endl;
      }
    }
    catch (VectorException & theVectorException)  {
      theVectorException.showErrors();
    }
    catch (Exception & theException) {
      theException.showErrors();
    }
    catch (...)  {
      cout << "Something went wrong!" << endl;
    }
    // End Try a vector size  > 20 but less the 100
  
    cout << "\nTry a vector size= 0\n";
    try  {
      Vector intVector(0);
      for (int j = 0; j < 100; j++) {
        intVector[j] = j;
        cout << "intVector[" << j << "] okay..." << endl;
      }
    }
    catch (VectorException & theVectorException)  {
      theVectorException.showErrors();
    }
    catch (Exception & theException) {
      theException.showErrors();
    }
    catch (...)  {
      cout << "Something went wrong!" << endl;
    }
    // End Try a vector size= 0
  
    cout << "\nTry a vector size  < 20 and >0\n";
    try  {
      Vector intVector(10);
      for (int j = 0; j < 100; j++) {
        intVector[j] = j;
        cout << "intVector[" << j << "] okay..." << endl;
      }
    }
    catch (VectorException & theVectorException)  {
      theVectorException.showErrors();
    }
    catch (Exception & theException) {
      theException.showErrors();
    }
    catch (...)  {
      cout << "Something went wrong!" << endl;
    }
    // End Try a vector size  < 20
    return 0;
  }

  When we run this application, the result will be:

  Try a vector size  > 20 but less the 100
  intVector[0] okay...
  intVector[1] okay...
  intVector[2] okay...
  intVector[3] okay...
  intVector[4] okay...
  intVector[5] okay...
  intVector[6] okay...
  intVector[7] okay...
  intVector[8] okay...
  intVector[9] okay...
  intVector[10] okay...
  intVector[11] okay...
  intVector[12] okay...
  intVector[13] okay...
  intVector[14] okay...
  intVector[15] okay...
  intVector[16] okay...
  intVector[17] okay...
  intVector[18] okay...
  intVector[19] okay...
  intVector[20] okay...
  intVector[21] okay...
  intVector[22] okay...
  intVector[23] okay...
  intVector[24] okay...
  intVector[25] okay...
  intVector[26] okay...
  intVector[27] okay...
  intVector[28] okay...
  intVector[29] okay...
  You got runtime error(s):
  The index is out of Boundary : 30
  
  Try a vector size= 0
  You got runtime error(s):
  The vector cannot be : 0
  The vector must be greater then 20 : 0
  
  Try a vector size  < 20 and >0
  You got runtime error(s):
  The vector must be greater then 20 : 10

  You can download this example here (needed tools can be found in the right menu on this page).