The implementation of std::basic_streambuf used private member variables
to manipulate the get and the put areas. Using public API functions is
equivalent but leads to code that is easier to understand, since the
public API functions are known more widely than our internal member
variables. Using the public API functions removes the need to map the
internal member variables back to get/put area manipulation functions in
one's head.
Finally, it also makes it easier to find subtle issues by instrumenting
accessor functions, which is impossible if the class uses the member
variables directly.
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