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 Section 7.12 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.
