unsetf/setf的问题?

C/C++ code

#include <iostream>using namespace std;int _tmain(int argc, _TCHAR* argv[]){    int i = 10;    cout << i << endl; // 输出10    cout.setf( ios_base::hex, ios_base::dec );    cout << i << endl; // 问题一:为什么输出10,而不是a呢?    cout.unsetf( ios_base::dec );    cout.setf( ios_base::hex );    cout << i << endl; // 输出a    cout.setf( ios_base::dec );    cout << i << endl; // 输出10    cout.setf( ios_base::hex, ios_base::dec );    cout << i << endl; // 输出a    return 0;}

C/C++ code

fmtflags setf(fmtflags fmtfl){    fmtflags old = m_flags;    m_flags |= fmtfl;    return old;}fmtflags setf(fmtflags fmtfl, fmtflags mask){    fmtflags old = m_flags;    m_flags &= ~mask;    m_flags |= (fmtfl & mask);    return old;}