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6.10 Deprecated Features

In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superseded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already.

While the list below is not exhaustive, it documents some of the options that are now deprecated:

These are two of the many ways for g++ to implement template instantiation. See Template Instantiation. The C++ standard clearly defines how template definitions have to be organized across implementation units. g++ has an implicit instantiation mechanism that should work just fine for standard-conforming code.
Previously it was possible to use an empty prototype parameter list to indicate an unspecified number of parameters (like C), rather than no parameters, as C++ demands. This feature has been removed, except where it is required for backwards compatibility See Backwards Compatibility.

The named return value extension has been deprecated, and is now removed from g++.

The use of initializer lists with new expressions has been deprecated, and is now removed from g++.

Floating and complex non-type template parameters have been deprecated, and are now removed from g++.

The implicit typename extension has been deprecated and will be removed from g++ at some point. In some cases g++ determines that a dependent type such as TPL<T>::X is a type without needing a typename keyword, contrary to the standard.