multiple threads writing to std::cout or std::cerr

Question

I have OpenMP threads that write to the console via cout and cerr. This of course is not safe, since output can be interleaved. I could do something like

#pr         


        

Answer 1

You can use an approach similar to a string builder. Create a non-template class that:

• offers templated operator<< for insertion into this object
• internally builds into a std::ostringstream
• dumps the contents on destruction

Rough approach:

 class AtomicWriter {
    std::ostringstream st;
 public:
    template  
    AtomicWriter& operator<<(T const& t) {
       st << t;
       return *this;
    }
    ~AtomicWriter() {
       std::string s = st.str();
       std::cerr << s;
       //fprintf(stderr,"%s", s.c_str());
       // write(2,s.c_str(),s.size());
    }
 };

Use as:

AtomicWriter() << "my variable: " << variable << "\n";

Or in more complex scenarios:

{
   AtomicWriter w;
   w << "my variables:";
   for (auto & v : vars) {
      w << ' ' << v;
   }
}  // now it dumps

You will need to add more overloads if you want manipulators, you can use write better than fprintf for the atomic write in the destructor, or std::cerr, you can generalize so that the destination is passed to the constructor (std::ostream/file descriptor/FILE*),