Now you are changing the subject. Allocation, which was the subject, happens the same way on either side of gcc's C++11 boundary. The C++11 break was required by the ISO Standard issued in 2011.
The Gcc and libstdc++ maintainers are extremely careful to maintain ABI compatibility between widely-separated releases, where permitted by the Standard. You can happily call between code compiled with gcc-6 and gcc-10, release or debug, optimized or not.
It is possible to deliberately vary ABI with compiler options, but nobody uses those for shared libraries they distribute, for obvious reasons, with the unique exception of choosing pre-C++11 vs post-C++11 ABI.
