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std::basic_ios<CharT,Traits>::copyfmt

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.

#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 event type `copy_event`, which is not defined	corrected to `copyfmt_event`
LWG 292	C++98	if `other` refers to the same object as `*this`, the member objects were still copied and the registered callbacks were still called	do nothing in this case

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