Standard C++ Library Reference ISO/IEC (VERSION3)

you guess wrong about its actual dependencies.) A Standard C header never includes another
standard header. A standard header declares or defines only the entities described for it in this
document.
Every function in the library is declared in a standard header. Unlike in Standard C, the standard
header never provides a masking macro, with the same name as the function, that masks the
function declaration and achieves the same effect.
All names other than operator delete and operator new in the C++ library headers
are defined in the std namespace, or in a namespace nested within the std namespace. You
refer to the name cin, for example, as std::cin. Note, however, that macro names are not
subject to namespace qualification, so you always write __STD_COMPLEX without a
namespace qualifier.
In some translation environments, including a C++ library header may hoist external names
declared in the std namespace into the global namespace as well, with individual using
declarations for each of the names. Otherwise, the header does not introduce any library names
into the current namespace.
The C++ Standard requires that the C Standard headers declare all external names in namespace
std, then hoist them into the global namespace with individual using declarations for each of
the names. But in some translation environments the C Standard headers include no namespace
declarations, declaring all names directly in the global namespace. Thus, the most portable way
to deal with namespaces is to follow two rules:
To assuredly declare in namespace std an external name that is traditionally declared in
<stdlib.h>, for example, include the header <cstdlib>. Know that the name might
also be declared in the global namespace.
●
To assuredly declare in the global namespace an external name declared in
<stdlib.h>, include the header <stdlib.h> directly. Know that the name might
also be declared in namespace std.
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Thus, if you want to call std::abort() to cause abnormal termination, you should include
<cstdlib>. And if you want to call abort(), you should include <stdlib.h>.
Alternatively, you can write the declaration:
using namespace std;
which assuredly hoists all library names into the current namespace. If you write this
declaration immediately after all include directives, you hoist the names into the global
namespace. You can subsequently ignore namespace considerations in the remainder of the
translation unit. You also avoid most dialect differences across different translation
environments.
Unless specifically indicated otherwise, you may not define names in the std namespace, or in
a namespace nested within the std namespace.