Destruction???

[exterminator]

Thanks Paul for explaining the undefined behaviour. However, I was not quite sure what a POD type meant. I looked for its definition and here I quote it.

A POD type is a C++ type that has an equivalent in C, and that uses the same rules as C uses for initialization, copying, layout, and addressing.

As an example, the C declaration struct Fred x; does not initialize the members of the Fred variable x. To make this same behavior happen in C++, Fred would need to not have any constructors. Similarly to make the C++ version of copying the same as the C version, the C++ Fred must not have overloaded the assignment operator. To make sure the other rules match, the C++ version must not have virtual functions, base classes, non-static members that are private or protected, or a destructor. It can, however, have static data members, static member functions, and non-static non-virtual member functions.

The actual definition of a POD type is recursive and gets a little gnarly. Here's a slightly simplified definition of POD: a POD type's non-static data members must be public and can be of any of these types: bool, any numeric type including the various char variants, any enumeration type, any data-pointer type (that is, any type convertible to void*), any pointer-to-function type, or any POD type, including arrays of any of these. Note: data-pointers and pointers-to-function are okay, but pointers-to-member are not. Also note that references are not allowed. In addition, a POD type can't have constructors, virtual functions, base classes, or an overloaded assignment operator.

The source of this definition is - C++ FAQ Lite - Marshall Cline.

So how do I ensure that my class is a POD type? That would mean that I would have to ensure that the object creation, copying, assignment are done by explicit non-virtual member functions (because in a C++ class we are by default provided with the 2 constructos) which seems to be a very wierd (and rare) thing. In that case I suppose we are restricted not to use classes instead we need to use C-type structs at most, in C++. So POD simply means the datatypes supported by C and hence I can conclude that when you are writing variadic functions in C++, its actually a C-code, else it would be an undefined behaviour. I am not sure if I am understanding the POD concept in relation to C++ classes that well.

[Paul McKenzie]

The definition is correct. It basically means that a class or struct must must be 'C' compatible to be POD. For C++ specfic POD types, you throw in static members and member functions into the mix. There really isn't much more to say beyond what the definition in the C++ FAQ states.

The one thing that a lot of programmers do (usually C programmers who are beginners using C++) is to use non-POD types as if they are POD types. The usage of them in printf() statements is just one mistake.

Basically, if you have a non-POD type, you must use C++ (not C) techniques to manipulate it. This means constructing it properly (not use malloc()), destroying it correctly, copying it using the appropriate operator = (not use memcpy()), use std::sort() to sort an array of non-POD types. Some other examples -- you can't use macros such as "offsetof()" on non-POD types, you can't use a non-POD type as the address of a buffer in an fwrite() call, etc.

Regards,

Paul McKenzie

[exterminator]

Thanks again Paul for the clarifications.

I was just going through an article that tells about the variadic functions and their alternatives in C++ that are:
1. Functions with Default Arguments
2. USage of Function Templates
3. Packing Function Arguments in a Container
4. Overloading operator <<
Here's the link if OPer wishes to go through the article - Informit. I hope all the concerns of sszd have been nicely addressed as are mine . Nice thread.

[sszd]

I would like to thank both Paul and exterminator for their time and input into this subject. And yes, I am quite satisified with the explanation.

I do have one additional question. Is there a site where one can read the actual ANSI standard documents? I've done some searching and can find nothing except some sites where you must pay to become a member before you are allowed to even browse the documents they have.

Thanks again.

[SuperKoko]

Here are the final drafts of ISO 98 C++ standard:
http://library.n0i.net/programming/c/cp-iso/

[Graham]

The actual standard only costs about $18 and is worth every penny. You can get it from here.