The top level program can use str.at(), but does every string function everywhere in all your libraries use str.at()? This is likely to be a particular problem

The top level program can use str.at(), but does every string function everywhere in all your libraries use str.at()? This is likely to be a particular problem for strings (as supposed to any other structure), due to the long and complicated history of strings in C and C++. There are likely to be many string functions across the codebase, and null termination in some contexts confuses the issue even more.