Перенес вывод логов в отдельный поток, функция log только формирует строку и помещает ее в очередь для логирования.

include/log_queue.h

@@ -0,0 +1,27 @@
+#ifndef LOG_QUEUE_H
+#define	LOG_QUEUE_H
+
+#include <queue>
+#include <thread>
+#include <mutex>
+#include <condition_variable>
+
+class log_queue
+{
+private:
+	std::thread _worker;
+	std::mutex _mutex;
+	std::condition_variable _cond;
+	std::queue<std::string> _queue;
+	
+private:
+	void worker_func();
+	
+public:
+	log_queue();
+	~log_queue();
+	void add_log(const std::string& logLine);
+};
+
+#endif	/* LOG_QUEUE_H */
+

include/logger.h

@@ -1,6 +1,8 @@
 #ifndef _LOGGER_H_
 #define _LOGGER_H_
 
+#include "log_queue.h"
+
 #include <sstream>
 #include <stdexcept>
 #include <iostream>
@@ -10,6 +12,7 @@ class logger
 {
 private:
     std::stringstream _logLine;
+	log_queue _queue;
 
 public:
     logger();

src/log_queue.cpp

@@ -0,0 +1,33 @@
+#include "log_queue.h"
+#include <iostream>
+
+log_queue::log_queue(): _worker(&log_queue::worker_func, this)
+{
+}
+
+log_queue::~log_queue()
+{
+    _worker.join();
+}
+
+void log_queue::worker_func()
+{
+    while(true)
+    {
+        std::unique_lock<std::mutex> lock(_mutex);
+        _cond.wait(lock, [this] { return !_queue.empty(); });
+        std::string line = _queue.front();
+        _queue.pop();
+        lock.unlock();
+        
+        // actually logging
+        std::cout << line << std::endl;
+    }
+}
+
+void log_queue::add_log(const std::string& logLine)
+{
+    std::lock_guard<std::mutex> lock(_mutex);
+    _queue.push(logLine);
+    _cond.notify_one();
+}

src/logger.cpp

@@ -17,6 +17,6 @@ void logger::log_internal(const char *s)
         _logLine << *s++;
     }
 
-    std::cout << _logLine.str() << std::endl;
+    _queue.add_log(_logLine.str());
 }
 

test/main.cpp

@@ -571,5 +571,8 @@ int main(int argc, char **argv)
     ulog.log("one: %, two: %", one, two);
     ulog.log("Hello world!");
     
+    int i = 0;
+    std::cin >> i;
+    
     return 0;
 }