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// Copyright (c) 2016 Egor Tensin <Egor.Tensin@gmail.com>
// This file is part of the "Egor's blog" project.
// For details, see https://github.com/egor-tensin/blog.
// Distributed under the MIT License.
#include <ctime>
#include <chrono>
#include <iostream>
#include <mutex>
#include <sstream>
#include <string>
#include <thread>
namespace
{
template <typename Derived>
class Singleton
{
public:
static Derived& get_instance()
{
std::call_once(initialized_flag, &initialize_instance);
return Derived::get_instance_unsafe();
}
protected:
Singleton() = default;
~Singleton() = default;
static Derived& get_instance_unsafe()
{
static Derived instance;
return instance;
}
private:
static void initialize_instance()
{
Derived::get_instance_unsafe();
}
static std::once_flag initialized_flag;
Singleton(const Singleton&) = delete;
Singleton& operator=(const Singleton&) = delete;
};
template <typename Derived>
std::once_flag Singleton<Derived>::initialized_flag;
class Logger : public Singleton<Logger>
{
public:
Logger& operator<<(const char*)
{
return *this;
}
private:
Logger()
{
std::this_thread::sleep_for(std::chrono::seconds{3});
}
~Logger() = default;
friend class Singleton<Logger>;
};
class Duke : public Singleton<Duke>
{
private:
Duke()
{
Logger::get_instance() << "started Duke's initialization";
std::this_thread::sleep_for(std::chrono::seconds{10});
Logger::get_instance() << "finishing Duke's initialization";
}
~Duke() = default;
friend class Singleton<Duke>;
};
std::mutex timestamp_mtx;
std::string get_timestamp()
{
std::lock_guard<std::mutex> lck{timestamp_mtx};
const auto tt = std::time(NULL);
return std::ctime(&tt);
}
void entered(const char* f)
{
std::ostringstream oss;
oss << "Entered " << f << " at " << get_timestamp();
std::cout << oss.str();
}
void exiting(const char* f)
{
std::ostringstream oss;
oss << "Exiting " << f << " at " << get_timestamp();
std::cout << oss.str();
}
void get_logger()
{
entered(__FUNCTION__);
Logger::get_instance();
exiting(__FUNCTION__);
}
void get_duke()
{
entered(__FUNCTION__);
Duke::get_instance();
exiting(__FUNCTION__);
}
}
int main()
{
std::thread t1{&get_duke};
std::thread t2{&get_logger};
t1.join();
t2.join();
return 0;
}
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