C++ – OpenMP – std::cout – Print with OpenMP

In this post I propose a simple class to print using cout in a program that uses OpenMP.
Of course you can use a mutex to protect the cout stream, but I prefer to use a buffer before printing as I show in this example.

The Code

#include <iostream>
#include <sstream>

class ParallelStream{
    std::ostringstream stdStream;
public:
    ParallelStream(){}
    template <class T>
    ParallelStream& operator<<(const T& inData){
        stdStream << inData;
        return *this;
    }
    std::string toString() const{
        return stdStream.str();
    }
};

Use it as …

// We create a temporary variable
// That is not very efficiency, but if you are printing you may not look to efficiency
std::cout << (ParallelStream() <<"I am " << omp_get_thread_num() << " working at section 3\n").toString();//<< std::endl;

//...

The next Method is old and depreciated

The code

#include <iostream>
#include <sstream>

#include <omp.h>

/**
* This class is a buffer to print using std::cout
* in parallel
*/
class Cout_MP{
private:
	// Buffer
	std::ostringstream oss;
	// private constructor
	Cout_MP(): oss(std::ostringstream::out){
	}
public:
	// to get a printer
	static Cout_MP Print(){
		return Cout_MP();
	}

	// for all types (except Action)
	template <class T>
	Cout_MP& operator<<(const T& input){
		oss << input;
		return *this;
	}

	// actions
	enum Action{
		ENDL,
		PRINT,
	};
	// for action type
	Cout_MP& operator<<(const Action& input){
		switch(input){
			// endl => append \n
			case ENDL:{
				oss << "\n";
			}
		    // endl or print => print
			case PRINT:{
				std::cout << oss.str();
				std::cout.flush();
				oss.clear();
			}break;
		}
		return *this;
	}
};

// Example
int _tmain(int argc, _TCHAR* argv[])
{
	std::cout << "Basic std::cout" << std::endl;
	#pragma omp parallel num_threads(4)
	{
		std::cout << "I want " << "to print many " << "things " << "thread : " << omp_get_thread_num() << std::endl;
	}

	std::cout << "With Cout_MP" << std::endl;
	#pragma omp parallel num_threads(4)
	{
		Cout_MP::Print() << "I want " << "to print many " << "things " << "thread : " << omp_get_thread_num() << Cout_MP::ENDL;
	}

	return 0;
}

Output

Basic std::cout
I want I want I want I want to print many to print many to print many to print m
any things things things things thread : thread : thread : thread : 0132

With Cout_MP
I want to print many things thread : 3
I want to print many things thread : 0
I want to print many things thread : 2
I want to print many things thread : 1
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