Multithreading Standard <thread> : C++

#include <iostream>
#include <thread>

template <class M = std::string, class E = int>
E log(M msg, E error = 0)
{
    std::cout << msg;
    return error;
}

class DisplayThread
{
    public:

        void operator()()
        {
            //std::cout << "Current thread  ID = " << std::this_thread::get_id() << "\n";
            for (int i = 0; i < 10; i++)
                log("<OBJECTS>\n");
        }
};

void thread_func()
{
    //std::cout << "Current thread  ID = " << std::this_thread::get_id() << "\n";
    for (int i = 0; i < 10; i++)
        log("<POINTER>\n");
}

int main()
{
    // use Function Pointer !
    std::thread threadFuncPointer(thread_func);

    // use Function Objects !
    std::thread threadFuncObjects((DisplayThread()));

    // use Lambda functions !
    std::thread threadFuncLambda([]()
    {
        //std::cout << "Current thread  ID = " << std::this_thread::get_id() << "\n";
        for (int i = 0; i < 10; i++)
            log("<LAMBDA>\n");
    });

    for (int i = 0; i < 100; i++)
        log("<MAIN>\n");

    std::cout << "Pointer ID = " << threadFuncPointer.get_id() << "\n";
    std::cout << "Objects ID = " << threadFuncObjects.get_id() << "\n";
    std::cout << "Lambda  ID = " << threadFuncLambda.get_id() << "\n";

    threadFuncPointer.join();
    threadFuncObjects.join();
    threadFuncLambda.join();

    std::cout << "Current thread  ID = " << std::this_thread::get_id() << "\n";

    return log("- End of main()\n");
}