Use << operator with own function to "override" cout - c++

Question

I know that there is a lot of topics about overloading << operator, but it seems to always be used in class in order to make it support << operator. Hope I would not be duplicate
What I want to do is (I think) quite different (and probably simpler).
I have a console app with 2 threads, both writing on console. I want to avoid them from smashing to each other in the console, so I use a mutex to prevent a thread from outputing when the other one is, so i've created the function :
void print(string s){
globals::console_mtx.lock();
cout << s << endl;
globals::console_mtx.unlock();
}
I would like it to be usable like this, regardless of the data type :
int i=5;
print << "Some text" << i << endl;
Is << overloading what I need ? what would be the simplest way to achieve it ?
Thanks

Answer 1

Is this what you're thinking about?:
struct print_t {};
static const print_t print = print_t();
print_t& operator<<(print_t& p, const std::string& s) {
globals::console_mtx.lock();
std::cout << s << endl;
globals::console_mtx.unlock();
return p;
}
NB. there are about a dozen ways to make it better, like having print as a function with unique type, etc. to make sure you don't run into static (de)initialization order fiasco (avoid the above in static deinitialization if possible). You might use the definition of cout in std as a cheat sheet.

Answer 2

The problem with just overloading << is that:
print << "Some text" << i << endl;
Actually calls print.operator<<() separately for "Some text", i, and endl (also you need to make sure to have overloads for int and endl). If you lock and unlock for each call, then another thread could get the lock before you are done with all 3 calls. If for instance thread 2 had
print << "Other text" << i+1 << endl;
Then you might end up with:
"Some TextOther Text65\n\n"
The simplest way to deal with it is to output to a stringstream first, and then use a normal print function to print the entire thing protected by a locked mutex:
stringstream ss;
ss <<"Some text" << i << endl;
print(ss.str());
If you insist on using streams directly, then streams handle this kind of thing by only actually writing when it receives an instruction to flush. endl automatically flushes the stream, which is one of the ways it differs from "\n".
You then implement a custom std::stringbuf that locks the mutex and outputs to console on sync(), then you construct an ostream for each thread to use as its personal printing output stream.
Example code:
#include <iostream>
#include <sstream>
#include <mutex>
#include <thread>
#include <string>
// Increase risk of race condition if one can be triggered.
char slow_get_ch(char ch)
{
for (unsigned int i = 0; i < 10000; ++i)
{
for (unsigned int j = 0; j < 10000; ++j)
{
}
}
return ch;
}
class print_buf : public std::stringbuf
{
std::mutex& mtx_;
public:
print_buf(std::mutex& mtx)
:
mtx_(mtx)
{
}
protected:
int sync() final
{
std::unique_lock<std::mutex> lck(mtx_);
std::string val = this->str();
std::cout << val;
this->str("");
return 0;
}
};
void print_worker(std::ostream* print_stream_ptr,char ch)
{
std::ostream& print = *print_stream_ptr;
// Print 5 lines of 20 times ch.
for (unsigned int i = 0; i < 5; ++i) {
for (unsigned int j = 0; j < 20; ++j) {
print << slow_get_ch(ch);
}
print << std::endl;
}
}
int main()
{
std::mutex print_mutex;
print_buf buf1(print_mutex);
print_buf buf2(print_mutex);
print_buf buf3(print_mutex);
std::ostream p1(&buf1);
std::ostream p2(&buf2);
std::ostream p3(&buf3);
std::thread t1(print_worker, &p1, 'a');
std::thread t2(print_worker, &p2, 'b');
std::thread t3(print_worker, &p3, 'c');
t1.join();
t2.join();
t3.join();
return 0;
}