C++ way of doing placement new on an mmap buffer

mmap_place_new.cpp

#include <cstdlib>
#include <cstdio>
#include <cstdint>
#include <sys/mman.h>
#include <new>
#include <atomic>

template<class T>
struct memory_ptr {
	std::atomic<bool> in_use;
	uint16_t tid;
	T obj;

	memory_ptr(): in_use(false), tid(0) {}
};

int main(void) {
	void* ptrmap = ::mmap( NULL, 50, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0);
	if (ptrmap == MAP_FAILED) {
		std::fprintf(stderr, "%s\n", "Got map failed!!");
		std::exit(1);
	}
	memory_ptr<int>* obj = new (ptrmap) memory_ptr<int>();
	std::printf("%zu\n", sizeof(memory_ptr<int>));
	obj->tid = 3;
	/*
	char* writer = (char*)ptrmap;
	writer[0] = 'e';
	writer[1] = 'w';
	writer[2] = '\0';
	std::puts(writer);*/
	::munmap(ptrmap, 50);
	return 0;
}