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

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< cpp‎ | io‎ | basic ios

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 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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