Concurrent cout formatting - c++

Question

This is an academic question.
sdt::mutex m;
typedef std::lock_guard<std::mutex> G;
void thread1(){
G g(m);
std::cout << std::setw(9);
std::cout << 3.14;
std::cout << std::endl;
}
void thread2(){
G g(m);
std::cout << std::setw(7);
std::cout << 3.14;
std::cout << std::endl;
}
My problem is that the formatting is bound to the output stream, so I need to set all the formatting options ever invented on my thread if I want to be sure about the output I produce. Which will or will not work next year.
Is there any way to reset formatting to default without setting everything by hand?
If not, what are the good workarounds?
For example should I create and keep an std::ostringstream locally on my thread and write oss.str() to std::cout?

Answer 1

I've used boost for brevity but you could write your own optional and state saver.
#include <mutex>
#include <iostream>
#include <iomanip>
#include <tuple>
#include <utility>
#include <boost/io/ios_state.hpp>
#include <boost/optional.hpp>
std::mutex m;
struct save_and_lock
{
boost::optional<boost::io::ios_all_saver> saver;
std::unique_lock<std::mutex> lock;
void init(std::ostream& os)
{
lock = std::unique_lock<std::mutex>(m);
saver.emplace(os);
os.flags(std::ios_base::fmtflags(0));
}
friend std::ostream& operator<<(std::ostream& os, save_and_lock&& s)
{
s.init(os);
return os;
}
};
void thread1(){
std::cout << save_and_lock() << std::setw(9) << 3.14 << std::endl;
}
void thread2(){
std::cout << save_and_lock() << std::setw(9) << 3.14 << std::endl;
}
This will work because the evaluation order of user-defined operator << is left to right.