Pedantic: What Is A Source File? What Is A Header?

Question

For the purposes of this question, I am interested only in Standard-Compliant C++, not C or C++0x, and not any implementation-specific details.

Questions arise from

Answer 1

My reading is that the standard headers, included by use of <> angle brackets, need not be actual files on the filesystem; e.g. an implementation would be free to enable a set of "built-in" operations providing the functionality of iostream when it sees #include <iostream>.

On the other hand, "source files" included with #include "xxx.h" are intended to be literal files residing on the filesystem, searched in some implementation-dependent manner.

Edit: to answer your specific question, I believe that "headers" are limited only to those #includeable facilities specified in the standard: iostream, vector and friends---or by the implementation as extensions to the standard. "Source files" would be any non-standard facilities (as .h files, etc.) the programmer may write or use.

Answer 2

The standard headers (string, iostream) don't necessarily have to be files with those names, or even files at all. As long as when you say

#include <iostream>

a certain list of declarations come into scope, the Standard is satisfied. Exactly how that comes about is an implementation detail. (when the Standard was being written, DOS could only handle 8.3 filenames, but some of the standard header names were longer than that)

Answer 3

Hmmm...

My casual understanding has been that the distinction between <> includes and "" includes was inherited from c and (though not defined by the standards) the de facto meaning was that <> searched paths for system and compiler provided headers and "" also searched local and user specified paths.

The definition above seem to agree in some sense with that usage, but restricts the use of "header" to things provided by the compiler or system exclusive of code provided by the user, even if they have the traditional "interface goes in the header" form.

Anyway, very interesting.

Answer 4

Isn't this saying that a header may be implemented as a source file, but there again may not be? as for "what is a source file", it seems very sensible for the standard not to spell this out, given the many ways that "files" are implemented.

Answer 5

As your quotes say: a header is something included using <>, and a source file is the file being compiled, or something included using "". Exactly where the contents of these come from, and what non-standard headers are available, is up to the implementation. All the Standard specifies is what is defined if you include the standard headers.

By convention, headers are generally system-wide things, and source files are generally local to a project (for some definition of project), but the standard wisely doesn't get bogged down in anything to do with project organisation; it just gives very general definitions that are compatible with such conventions, leaving the details to the implementation and/or the user.

Nearly all of the standard deals with the program after it's been preprocessed, at which time there are no such things as source files or headers, just the translations units that your last quote defines.