Thread-safe singleton

ts-singleton.cpp

#include <mutex>
#include <thread>
#include <iostream>

struct Singleton
{
    static Singleton& instance()
    {
        std::lock_guard<std::mutex> lock(m);
        static Singleton s;
        return s;
    }
    
    Singleton() = default;
    Singleton(Singleton const&) = delete;
    Singleton& operator=(Singleton const&) = delete;
private:
    static std::mutex m;
};

std::mutex Singleton::m;

int main()
{
    Singleton *p1, *p2;
    std::thread t1([&] { p1 = &Singleton::instance(); });
    std::thread t2([&] { p2 = &Singleton::instance(); });
    t1.join();
    t2.join();
    
    std::cout << p1 << " " << p2;
}

p1 and p2 point to the same singleton. Had a mutex not been used, the initialization of the static singleton object couldn't been accessed concurrently leading to the creation of two different singleton objects.