std::strstream::rdbuf
From cppreference.com
Returns a pointer to the associated std::strstreambuf, casting away its constness (despite the const qualifier on the member function).
Parameters
(none)
Return value
Pointer to the associated std::strstreambuf, with constness cast away.
Example
Run this code
#include <strstream>
int main()
{
const std::strstream buf;
std::strstreambuf* ptr = buf.rdbuf();
}