Warning C4099: type name first seen using 'class' now seen using 'struct' (MS VS 2k8)

Question

Is this warning anything to worry about? I've read that it can cause erratic behaviour?

It's an example I'm trying to compile, could someone explain to me why the author declares the object as a class but then typedef's it to a structure? Is it perfectly normal to do so if the class is POD?

Thanks.

Answer 1

Although this is considered bad practice, I think there should be no problem mixing class definition and struct declaration, as they are basically the same data type. The main difference is that struct members are by default public, contrary to class members which are private, but otherwise the memory layout is identical.

Answer 2

In c++ the only difference between a class and a struct is that class's members and functions are private by default, while in a struct they're by default public; so, the fact that the class is POD should not make any difference here.
I would guess that this warning comes from code maintenance (definition updated somewhere but not somewhere else), and fix the code so that the warning disappears (e.g. using class in the typedef).

Answer 3

This warning appears when you have a one type declaration that contradicts another (one says "class", the other says "struct"). Given the one definition rule, all declarations except for at most one must be forward declarations. The warning will generally indicate that a forward declaration of a type is wrong and is usually a simple typo and should be fixed. In this case there should be no side effects, but you really should fix it.

There can be, however, some very nasty things happen if you have type name clashes (perhaps caused by using "using namespace" clauses or global namespace pollution). These warnings could be indicating that you are mixing headers from two different libraries and the type names have clashes. Code compiled under these conditions could do some very unexpected things.

My advice - understand why the warning has appeared and fix it. If the warning is in a third party product, insist that they fix it.

Answer 4

Just to bring the comment by MSalters against this post above to the top level. I have had several hard to find linker errors as a result of VC using the 'class' or 'struct' keyword in its mangling of names.

If you don't expect it to be a problem you can be left scratching your head for hours!

Answer 5

Richard Corden is correct - there is a reason MS has this warning. Decorated (mangled) names include which class key (struct or class) a type name is. If a function or method that takes some object as an argument or returns that object is referenced somewhere when the wrong class key is visible, you will not get a compile error but the linker will complain because the decorated names differ. The linker error only shows the symbol it's looking for, and it's easy to overlook the class key mismatch there, so the earlier, more detailed compiler warning is valuable. It's still possible that the two versions don't appear in the same compilation unit, of course, and you will probably be scratching your head for a while if you believe that the only difference is default member visibility.

Answer 6

In my case, generated files (from CORBAL IDL) has the keyword struct and I did fwd declared it as class. Do I need to fix my usage?

Answer 7

I discuss this warning in depth in my blog posting "Is C4099 really a sillywarning?". My conclusion is that it is best turned off. :-) Well, at least for me.