std::basic_stringbuf<CharT,Traits,Allocator>::setbuf (3) - Linux Manuals
virtual std::basic_streambuf<CharT, Traits>* setbuf( char_type* s, std::streamsize n )
If s is a null pointer and n is zero, this function has no effect.
Otherwise, the effect is implementation-defined: some implementations do nothing, while some implementations clear the std::string member currently used as the buffer and begin using the user-supplied character array of size n, whose first element is pointed to by s, as the buffer and the input/output character sequence.
This function is protected virtual, it may only be called through pubsetbuf() or from member functions of a user-defined class derived from std::basic_stringbuf.
s - pointer to the first CharT in the user-provided buffer or null
n - the number of CharT elements in the user-provided buffer or zero
The deprecated stream buffer std::strstreambuf or the boost.IOStreams device boost::basic_array may be used to implement I/O buffering over a user-provided char array in portable manner.
Test for the stringstream's setbuf functionality
// Run this code
pubsetbuf (public member function of std::basic_streambuf<CharT,Traits>)