Multi threading c++ unexpected code output [closed]

I am studying multi-threading in c++ with a simple code.

#include <thread>
#include <iostream>


void foo()
{

    std::cout << "test" << std::endl;

}

int main()
{
    std::cout << "Starting Thread 1..." << std::endl;
    std::thread t1(foo);
    t1.join();

    return 0;
}

And this code outputs

Starting Thread 1...
release
get()
this->_Myptr
release
get()
this->_Myptr
test

what does release, get(), this->_Myptr mean? I try to google for an answer but had no luck.