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### 6.12 Backwards Compatibility

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. See 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.