<iostream> vs. <iostream.h> vs. "iostream.h"

As the person on the standards committee (X3J16) who proposed leaving off the .h, my original intent was to settle the debate over .h, .H, .hpp, .hxx, or .h++ file extensions; or a desire by some that there be no implication in the standard that this was the name of a file on disk in order to allow an IDE to pull pre-compiled header information out of somewhere internal, like a resource file or even the guts of the compiler.

While Unix considered the filename to be a single string and did not actually recognize the concept of an extension, DEC operating systems had a tradition of separating the name from the extension, and supplying the "default extension" if it was omitted in particular contexts. That's where I got the idea from of leaving it up to the implementation to use whatever extension the implementation wanted to use, and it allowed the implementation to not even have this a file on disk. (I was DEC's representative on the committee at the time.)

Differentiating between the standard and the pre-standard headers was an added benefit.

In short:

iostream.h is deprecated—it is the original Stroustrup version. iostream is the version from the standards committee. Generally, compilers point them both to the same thing, but some older compilers won't have the older one. In some odd cases, they will both exist and be different (to support legacy code) and you then must be specific.

"" versus <> simply means check the local directories for the header before going to the library (in most compilers).

Here is a decent link article.

To summarize, the reason given:

The version of the iostream library that the Standards Committee produced was quite a bit different from the CFront implementation. {snip}

To ease transition, the C++ Standards Committee declared that code including the standard C++ headers would use include directives that lack an extension. This allowed compiler vendors to ship the old style C++ library headers with the .h extension and the new style headers without.

An advantage of not using the .h version:

There are several reasons why new code should be written using the extensionless version of the header files instead of the .h forms. The first is the unpredictability of such code when compiled on modern compilers. As previously mentioned, the result of using the .h headers is implementation specific. And as time goes by, the chance that a given compiler will have the old style library available decreases.