C++ human-readable thread id

thread_id.cpp

#include <mutex>
#include <thread>
#include <unordered_map>

std::size_t get_thread_id() noexcept {
    static std::size_t thread_idx = 0;
    static std::mutex thread_mutex;
    static std::unordered_map<std::thread::id, std::size_t> thread_ids;

    std::lock_guard<std::mutex> lock(thread_mutex);
    std::thread::id id = std::this_thread::get_id();
    auto iter = thread_ids.find(id);
    if (iter == thread_ids.end()) {
        iter = thread_ids.insert(std::pair<std::thread::id, std::size_t>(id, thread_idx++)).first;
    }
    return iter->second;
}