[earenndil]

That's not exactly the same. What you're referring to is the library ABI for the c++ standard library. Every library which can be linked to dynamically has its own ABI. The language ABI, on the other hand, describes how every library's ABI is defined, describing things like layout and name mangling. So if, for instance, the language ABI were amended to say that class members are arranged in alphabetical order in memory, then capacity_ will always be at offset 0, data_ at offset 8, and len_ at offset 16; if you change the order of declarations then, the library's ABI won't change. But if the library were compiled with an old compiler that targeted the old ABI, then it would put data_ first, followed by len_, then capacity_. So if you then compiled a new piece of code with a new compiler targeting the new ABI, but linked against it the library, there would be a mismatch.