Are unspecified and undefined behavior required to be consistent between compiles of the same program with the same compiler in the same environment?

Question

Let\'s pretend my program contains a specific construct the C++ Standard states to be unspecified behavior. This basically means the implementation has to do something reasonabl

Answer 1

If it's undefined behaviour then by it's very nature what will happen is undefined, you can't rely on it to be the same under any circumstances.

Unspecified behaviour on the other hand is something left up to individual vendors to decide how to implement, if there are ambiguities in the language spec for example. This will be consistent between compiles and runs, but not necessarily between different vendors. So, for example, relying on unspecified behaviour when you only build using Visual Studio is fine but if you try and port the code to gcc it may fail or produce a different behaviour than you are expecting.