Why C++11 compiler support still requires a flag?

Question

I understand that experimental features of a programming language should not be enabled by default so I welcome the flags -std=c++0x and -std=c++1y. However C++11 is now standard since a couple of years. Why compilers still require -std=c++11 to enable the support for its features?

Answer 1

C++11 has been standard for a couple of years, but a compiler isn't going to switch its default mode to C++11 until:

• At an absolute minimum, C++11 support is complete in that compiler and the libraries it uses. And also stable, if the compiler writer has any concern at all for reliability.
• Preferably, a major version number increase in the compiler, since C++11 is not fully backward-compatible to C++03.
• Ideally, on a well-known schedule so that users can prepare for the change.

Basically, lots of people (and makefiles) rely on the compiler being a conforming C++03 compiler, or at least on its non-conformance being known. Since C++11 introduces new instances of non-conformance with C++03, the change is potentially traumatic.

Arguably anyone relying on C++03 should have specified an option to say so, and changes to the default mode would make no difference to them. But once you've documented your compiler's default, people are going to rely that, either intentionally or otherwise.

For gcc in particular, the 4.8.2 man page says that "support for C++11 is still experimental". So I think ultimately the answer to your question may be that it takes more than 2 years to properly implement C++11, even starting from all the work done with draft standards.