There are definitely backwards-incompatible changes in the things that were added since C++98. For one, there's a whole set of new keywords that's going to brea

There are definitely backwards-incompatible changes in the things that were added since C++98. For one, there's a whole set of new keywords that's going to break any code that used them as identifiers. Then there's a keyword that was re-purposed to do something totally different: in C++, auto used to be a storage duration specifier. In C++11 it is used to indicate type inference. There were also changes that, while not breaking source compatibility, basically required implementations to break binary compatibility going from C++98 to C++11. This in particular resulted in a gigantic mess. With regard to breaking changes the C++ committee is not anywhere near as conservative as the one for C.