Restore the state of std::cout after manipulating it

you need to #include <iostream> or #include <ios> then when required:

std::ios_base::fmtflags f( cout.flags() );

//Your code here...

cout.flags( f );

You can put these at the beginning and end of your function, or check out this answer on how to use this with RAII.

The Boost IO Stream State Saver seems exactly what you need. :-)

Example based on your code snippet:

void printHex(std::ostream& x) {
    boost::io::ios_flags_saver ifs(x);
    x << std::hex << 123;
}

Note that the answers presented here won't restore the full state of std::cout. For example, std::setfill will "stick" even after calling .flags(). A better solution is to use .copyfmt:

std::ios oldState(nullptr);
oldState.copyfmt(std::cout);

std::cout
    << std::hex
    << std::setw(8)
    << std::setfill('0')
    << 0xDECEA5ED
    << std::endl;

std::cout.copyfmt(oldState);

std::cout
    << std::setw(15)
    << std::left
    << "case closed"
    << std::endl;

Will print:

case closed

rather than:

case closed0000