std::basic_ios::copyfmt(3)

NAME
std::basic_ios::copyfmt - std::basic_ios::copyfmt

Synopsis
   basic_ios& copyfmt( const basic_ios& other );
   If other refers to the same object as *this, has no effects. Otherwise, copies the
   state of the stream other into *this. This is done in the following sequence:
   1) Calls every callback registered by register_callback() passing erase_event as
   parameter.
   2) Copies all member objects from other to *this except for rdstate(), the exception
   mask, and rdbuf(). In particular, makes copies of the locale, the formatting flags,
   the contents of the arrays std::ios_base::iword and std::ios_base::pword (but not
   the iword and pword pointers themselves), the callbacks, and the tied stream.
   3) Calls every callback registered by register_callback() passing copyfmt_event as
   parameter.
   4) Copies the exception mask from other to *this as if by calling
   exceptions(other.exceptions()).

Parameters
   other - another stream to use as source

Return value
   *this

Notes
   The second pass through the callbacks may be used to deep-copy the user-defined
   objects pointed to by the pointers in std::ios_base::pword.
   copyfmt() may be used to save and restore the state of a stream. Boost provides a
   more fine-grained I/O state savers library for the same purpose.

Example
   Makes the std::ofstream object "out" behave exactly like std::cout, including
   formatting, tie() to std::cin, etc. // Run this code
 #include <bitset>
 #include <climits>
 #include <fstream>
 #include <iostream>
 int main()
 {
     std::ofstream out;
     out.copyfmt(std::cout); // copy everything except rdstate and rdbuf
     out.clear(std::cout.rdstate()); // copy rdstate
     out.basic_ios<char>::rdbuf(std::cout.rdbuf()); // share the buffer
     out << "Hello, world\n";
     auto bin = [](std::ios_base::fmtflags f)
     {
         return std::bitset<sizeof(std::ios_base::fmtflags) * CHAR_BIT>
             { static_cast<unsigned long long>(f) };
     };
     std::ofstream out2;
     std::cout << "1) out2.flags(): " << bin(out2.flags()) << '\n';
     std::cout << "2) cout.flags(): " << bin(std::cout.flags()) << '\n';
     std::cout.setf(std::ios::hex | std::ios::fixed | std::ios::boolalpha);
     std::cout << "3) cout.flags(): " << bin(std::cout.flags()) << '\n';
     out2.copyfmt(std::cout); // copy everything except rdstate and rdbuf
     std::cout << "4) out2.flags(): " << bin(out2.flags()) << '\n';
 }

Possible output:
 Hello, world
 1) out2.flags(): 00000000000000000001000000000010
 2) cout.flags(): 00000000000000000001000000000010
 3) cout.flags(): 00000000000000000001000000001111
 4) out2.flags(): 00000000000000000001000000001111
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   LWG 256 C++98 step 3 called the registered callbacks with the corrected to
                      event type copy_event, which is not defined copyfmt_event
                      if other refers to the same object as *this, the
   LWG 292 C++98 member objects do nothing
                      were still copied and the registered callbacks in this case
                      were still called