Exceptions

  General Description
 

• A way of dealing with errors
• Calling functions must deal with
• Caller cannot simply ignore them

Exceptions

    General Description

    They are used to deal with errors.

    If:

• something is passed in that is incorrect
• unusual data is returned by a function called

instead of returning a garbage value, from a function, that can be ignored,
throw an exception, because it must be dealt with.

Exceptions
 

    Behavior
 

• Exceptions, once thrown immediately exit all functions, until caught

Exceptions

    Behavior (continued)
 

• Exceptions, once thrown immediately exit all functions, until caught
• Immediately exit current function, no matter what!
• If the function, which called the function that threw the exception does not catch the exception, then the exception will exit this calling function.
• Behavior continues until the exception is caught, or program execution is terminated.


Shawn Stoffer -- CS151 Spring 2001

Exceptions

    Behavior (continued)
 

• When using exceptions, you must be careful, as if you do not catch them, they will cause your program to terminate.

Exceptions

    Syntax
 

• throw <exception variable>;
• try {  <statements> } catch (<exception variable>) { <statements> }

Exceptions

    Syntax

        throw <exception variable>;

        You can 'throw' anything, a value (1, 4.5, 9.0, etc...), or a variable.

        Whatever you 'throw' is passed with the exception through the functions which called the offending function.  (read: offending function = function that threw the exception).

Exceptions

    Syntax
        Example

int getElement(int rows) {
    int i = 5;
    if (rows > MAXROWS) {
        throw 5;
    }
    return table[rows];
}

Exceptions

    Syntax
        Example

enum Excepts
{
    OutOfRangeException
};
int getElement(int rows)
{
    Excepts i = OutOfRangeException;
    if (rows > MAXROWS)
    {
        throw i;
    }
    return table[rows];
}

Exceptions

    Syntax

    try { <statements> } catch (<exception variable>) { <statements> }
 

Slide 11:

Exceptions

    Syntax

    try { <statements> } catch (<exception variable>) { <statements> }
 

• try { <statements> }
• statements are normal C++ statements that you want to try, but might throw an exception
• because they might throw an exception, you want to put them in a try 'block'

Exceptions

    Syntax

    try { <statements> } catch (<exception variable>) { <statements> }
 

• catch (<exception variable>)
• looks like a function description
• i.e.   catch (int e)

Exceptions

    Syntax

    try { <statements> } catch (<exception variable>) { <statements> }
 

• catch (<exception variable>)
• looks like a function description
• i.e.   catch (Excepts e)

Exceptions

    Syntax

    try { <statements> } catch (<exception variable>) { <statements> }
 

• catch (<exception variable>)
• looks like a function description
• but will only catch exceptions which are of the type defined...
• so, if the <statements> of the try block throw an int, then you must catch an int.

Exceptions

    Syntax

    try { <statements> } catch (<exception variable>) { <statements> }
 

• catch (<exception variable>)
• looks like a function description
• you may have multiple catch statements, for different types of exceptions:
• i.e.

catch (int e) { /* do something */ }
catch (float e) { /* do something else */ }

Exceptions

    Syntax

    try { <statements> } catch (<exception variable>) { <statements> }
 

• catch (<exception variable>) { <statements> }
• <statements> are normal C++ statements
• what to do in case this exception is thrown...

Exceptions

    Syntax  (example)

    try { <statements> } catch (<exception variable>) { <statements> }
 

void afunc(int e) throw(int)
{
    if (e > 5)
    {
        throw -1;
    }
}
 

int i = 9;
try {
    afunc(i);
} catch (int e) {
    if (e == -1) {
        cout << "We encountered the exception for an invalid value!" << endl;
        return;
    }
} catch (...) {
    cout << "An unknown exception was found!!" << endl;
    throw;
}

Exceptions

    Syntax (example)

    try { <statements> } catch (<exception variable>) { <statements> }
 

• void afunc(int e) throw(int)
• What is throw(int)???
• This tells the compiler that the function may throw an exception, and is absolutely necessary for valid C++.
• Also warns someone using that function what types of exceptions that they have to expect to catch.

Exceptions

    Syntax (example)

    try { <statements> } catch (<exception variable>) { <statements> }
 

• void afunc(int e) throw(int)
• What is throw(int)???
• This tells the compiler that the function may throw an exception, and is absolutely necessary for valid C++.
• Also warns someone using that function what types of exceptions that they have to expect to catch.

Exceptions

    Syntax (example)

    try { <statements> } catch (<exception variable>) { <statements> }
 

• catch (...) {
• What is (...)???
• This tells C++ to catch ANY other type of exception that is thrown by the code.
• Though, then you have to deal with it...

Exceptions

    Syntax (example)

    try { <statements> } catch (<exception variable>) { <statements> }

• throw;
• What is 'throw;' throwing???
• If a throw is by itself in a catch block... then throw will re-'throw' the exception that it caught...
• so if you don't know what to do, or simply want to acknowledge the exception, but can't deal with it, then re-'throw' it.