a question about exceptions

[salah_saleh]

hey!
I have some questions regarding exceptions.
concerning this code:

try
15     {
16        cout << "  Function throwException throws an exception\n";
17        throw exception(); // generate exception
18     } // end try
19     catch ( exception & ) // handle exception
20     {
21        cout << "  Exception handled in function throwException"
22           << "\n  Function throwException rethrows exception";
23        throw; // rethrow exception for further processing
24     } // end catch

the book states that in line 17 exception is an instance of class exception, that I can't understand, it looks to me more like a function, I would write it that way "throw exception exp" as an example.
Thanks.

[mike_2000_17]

>> it looks to me more like a function

Or like the construction of an object, which it is.

If I have a class named "my_class", I can create an object of that class with "my_class()", which means the object is default-constructed and doesn't have a name (a temporary). So, the "exception()" just creates a temporary object of class "exception" that is default-constructed. The "throw" keyword takes whatever you give it (e.g. object) and propagates it (or carries it) back to the first try-block it encounters, where it will attempt to match it to one of the catch-statements.

You need to provide the throw statement with something to throw, it is most usual to give it a temporary object since there is usually no point in creating the exception object before throwing it, but in theory you could also do:

try
{
   cout << "  Function throwException throws an exception\n";
   exception e; //create exception object.
   throw e;     // and throw it.
} // end try
catch ( exception & ) // handle exception
{
   cout << "  Exception handled in function throwException"
        << "\n  Function throwException rethrows exception";
   throw; // rethrow exception for further processing
} // end catch

But why take two lines of code to do what one lines does equally well:

throw exception(); //this is pretty much the same as above (but less wasteful).

[salah_saleh]

It's the first time to be exposed to something like that, actually that is the first time to know that I can invoke the constructor directly by this way.
it seems an interesting topic for me and I want to know more about it, when to use such method and so on, could you please tell where can I read more about this topic?

[mike_2000_17]

Usually, I refer beginners to Marshall Cline's C++ FAQ page for general explanation of just about everything that is fundamental to C++.

>>actually that is the first time to know that I can invoke the constructor directly by this way.

It is not a direct invocation of the constructor. It is creating a temporary object, that's not the same thing. A constructor cannot be invoked directly (as a function call), it is always associated with the creation of an object (that's why it is called a "constructor", because it is used during the construction of an object).

I presume you are aware of this syntax:

int a(42);  //creates a local variable, of type 'int', called 'a', and initialized to '42'.

Now, if you just want to create a local variable but that is temporary (which means it does not need to have a name), than you just omit the name from the syntax, and get:

int(42);

Now, if you have a class that takes no parameters to one of its constructors (the default constructor), then the equivalent to the above for that custom class with default initialization is:

my_class();

That's about all there is to know about this. Don't know what else to say.

>>it seems an interesting topic for me

I can't be sure what "interesting topic" you are talking about. If you are talking about the above matter about creating temporary objects, then I can hardly see how you would find it "an interesting topic", it's just a basic syntax rule of C++. If you are talking about exception handling, then that is certainly a very vast and interesting subject. This FAQ section is one of the best resources I know about exceptions and error handling (many authors tend to give huge emphasis on OOP and standard libraries, they often tend to neglect the subject of exception-handling, partly because it is harder and less clear-cut than the other subjects). Just google for "exception-handling in C++" and you will find plenty of interesting articles.

[salah_saleh]

Thanks. I was confused a little bit, I thought it is a calling to constructor directly, but now I got it. thanks again for the further explanation.