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Why is C++ backward compatibility important / necessary?

As far as understand it is a well-established opinion within the C++ community that

  • C is an obsolete language that was useful 20 years ago but cannot support many modern good programming practices, or even encourages bad practices;
  • certain features that were typical of C++ (C with classes) during the nineties are also obsolete and considered bad practice in modern C++ (e.g., new and delete should be replaced by smart pointer primitives).

In view of this, I often wonder why backward compatibility with C and obsolete C++ features is still considered important: to my knowledge there is no 100% compatibility, but most of C and C++ are contained in C++11 as a subset.

Of course, there is a lot of legacy code and libraries (possibly containing templates) that are written using a previous standard of the language and which still need to be maintained or used in connection with new code.

Nevertheless, maybe it would still be possible to drop obsolete C and C++ features (e.g. the mentioned new / delete) from a future C++ standard so that it is impossible to use them in new code.

In this way, old and dangerous programming practices would be quickly banned from new code, and modern, better programming practices would be enforced by the compiler. Legacy code could still be maintained using separate compilation (having C alongside C++ source files is already a common practice). Developers would have to choose between one compiler supporting the old-style C++ that was common during the nineties and a compiler supporting the modern C++? style (the question mark indicates a future, hypothetical revision). Only mixing the two styles would be forbidden.

Would this be a viable strategy for encouraging the adoption of modern C++ practices? Are there conceptual reasons or technical problems (e.g. compiling existing templates) that make such a change undesirable or even impossible? Has such a development been proposed in the C++ community. If there has been some extended discussion on the topic, is there any material on-line?