Retrieving flags in c++ streams

How can i make use of flags in c++ streams? I know of ios_base::flags(), but when i cout them or compare them, they dont change values even with a new flag. a simple program:

#include <iostream>
using namespace std;

int main(){
     cout << cout.flags() << endl;//4098
     cout << std::hex << cout.flags() << endl;// 0x1002
     retur开发者_JAVA技巧n 0;
}

doesnt change the outputted default value (at least for me) of 4098.

my ultimate goal is to compare the stream to flags to see which are set, NOT TO SET NEW ONES. can anyone show me an example of how to do this?

---

With this code:

cout << std::hex << cout.flags() << endl;

The compiler is allowed to evaluate it in this order:

ios_base::fmtflags f = cout.flags();  // store value before applying std::hex
cout << hex;
cout << f;
cout << endl;

So you're not guaranteed to "see" flag changes this way. However, it is not Undefined Behavior.

The flags are a "bitmask type", which is defined to have certain properties – the actual type used is implementation-defined, but integers, enums, and std::bitsets are possibilities. You can use the normal bit-manipulation operators: ^, &, |, and ~:

bool is_hex(std::ios_base &s) {
  return (s.flags() & s.basefield) == s.hex;
}
// is_oct is identical, except with s.oct

// Nothing set in basefield means "determine base from input" for istreams,
// and ostreams use base 10.  This makes is_dec harder to write.

bool is_anybase(std::istream &s) {
  return (s.flags() & s.basefield) == 0;
}

bool is_dec(std::istream &s) {
  std::ios_base::fmtflags base = s.flags() & s.basefield;
  return base == dec;
}
bool is_dec(std::ostream &s) {
  std::ios_base::fmtflags base = s.flags() & s.basefield;
  return (base == dec) || (base == 0);
}
// Purposeful overload ambiguity on std::iostream.
// In 0x, we could write:
bool is_dec(std::iostream &s) = delete;

For an example, this is how std::hex works:

std::ios_base& hex(std::ios_base &s) {
  s.setf(s.hex, s.basefield);
  return s;
}

Where setf does:

ios_base::fmtflags fmtflags = s.hex;    // first parameter
ios_base::fmtflags mask = s.basefield;  // second parameter
s.flags((s.flags() & ~mask) | (fmtflags & mask));