|  |  16.2.12 Standard library headers 
Newer C++ implementations provide a new set of standard library header
files.  These are distinguished from older incompatible header files by
their filenames--the new headers omit the conventional `.h'
extension.  Classes and other declarations in the new headers are placed
in the stdnamespace.  Detecting the kind of header files present
on any given system is an ideal application of Autoconf.  For instance,
the header `<vector>' declares the classstd::vector<T>.
However, if it is not available, `<vector.h>' declares the classvector<T>in the global namespace. 
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