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(gcc-300.info)Backwards Compatibility

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Backwards Compatibility
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   Now that there is a definitive ISO standard C++, G++ has a
specification to adhere to.  The C++ language evolved over time, and
features that used to be acceptable in previous drafts of the standard,
such as the ARM [Annotated C++ Reference Manual], are no longer
accepted.  In order to allow compilation of C++ written to such drafts,
G++ contains some backwards compatibilities.  _All such backwards
compatibility features are liable to disappear in future versions of
G++._ They should be considered deprecated Note: Deprecated Features.

`For scope'
     If a variable is declared at for scope, it used to remain in scope
     until the end of the scope which contained the for statement
     (rather than just within the for scope).  G++ retains this, but
     issues a warning, if such a variable is accessed outside the for
     scope.

`implicit C language'
     Old C system header files did not contain an `extern "C" {...}'
     scope to set the language.  On such systems, all header files are
     implicitly scoped inside a C language scope.  Also, an empty
     prototype `()' will be treated as an unspecified number of
     arguments, rather than no arguments, as C++ demands.