Scope of C libraries in C++ - <X.h> vs <cX>

The C++ Programming Language : Special Edition states on page 431 that...

For every header < X.h > defining part of the C standard library in the global namespace and also in namespace std, there is a header < cX > defining the same names in the std namespace only.

However, when I use C headers in the < cX > style, I don't need to qualify the namespace. For example...

#include <cmath>
void f() {
  double var = sqrt( 17 );

This would compile fine. Even though the book says that using the < cX > header defines names in the std namespace only, you are allowed to use those names without qualifying the namespace. What am I missing here?

P.S. Using the GNU.GCC compiler


Stephan T. Lavavej, a member of the MSVC team, addresses the reality of this situation (and some of the refinements to the standard) in this comment on one of his blog postings (

> also, <cstddef>, <cstdlib>, and std::size_t etc should be used!

I used to be very careful about that. C++98 had a splendid dream wherein <cfoo> would declare everything within namespace std, and <foo.h> would include <cfoo> and then drag everything into the global namespace with using-declarations. (This is D.5 [depr.c.headers].)

This was ignored by lots of implementers (some of which had very little control over the C Standard Library headers). So, C++0x has been changed to match reality. As of the N2723 Working Paper, , now <cfoo> is guaranteed to declare everything within namespace std, and may or may not declare things within the global namespace. <foo.h> is the opposite: it is guaranteed to declare everything within the global namespace, and may or may not declare things within namespace std.

In reality and in C++0x, including <cfoo> is no safeguard against everything getting declared in the global namespace anyways. That's why I'm ceasing to bother with <cfoo>.

This was Library Issue 456, .

(C++0x still deprecates the <foo.h> headers from the C Standard Library, which is hilarious.)

I'm in 100% agreement with Lavavej, except I never tried to be very careful about using the <cfoo> style headers even when I first started using C++ - the standard C ones were just too ingrained - and there was never any real world problem using them (and apparently there was never any real world benefit to using the <cfoo> style headers).

The rule for the C libraries differs from C++ libraries for namespaces

gcc interprets the standard in Gcc docs as

The standard specifies that if one includes the C-style header (<math.h> in this case), the symbols will be available in the global namespace and perhaps in namespace std:: (but this is no longer a firm requirement.) One the other hand, including the C++-style header (<cmath>) guarantees that the entities will be found in namespace std and perhaps in the global namespace.

In the draft C0X++ spec it says in section Headers

It is unspecified whether these names are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations

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