why do I need std::condition_variable?

You can code this either way:

1. Using atomics and a polling loop.
2. Using a condition_variable.

I’ve coded it both ways for you below. On my system I can monitor in real time how much cpu any given process is using.

First with the polling loop:

#include <atomic>
#include <chrono>
#include <iostream>
#include <thread>

std::atomic<bool> is_ready(false);

void
test()
{
    std::this_thread::sleep_for(std::chrono::seconds(30));
    is_ready.store(true);
}

int
main()
{
    std::thread t(test);
    while (!is_ready.load())
        std::this_thread::yield();
    t.join();
}

For me this takes 30 seconds to execute, and while executing the process takes about 99.6% of a cpu.

Alternatively with a condition_variable:

#include <chrono>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread>

bool is_ready(false);
std::mutex m;
std::condition_variable cv;

void
test()
{
    std::this_thread::sleep_for(std::chrono::seconds(30));
    std::unique_lock<std::mutex> lk(m);
    is_ready = true;
    cv.notify_one();
}

int
main()
{
    std::thread t(test);
    std::unique_lock<std::mutex> lk(m);
    while (!is_ready)
    {
        cv.wait(lk);
        if (!is_ready)
            std::cout << "Spurious wake up!\n";
    }
    t.join();
}

This has the exact same behavior except that during the 30 second execution, the process is taking 0.0% cpu. If you’re writing an app that might execute on a battery powered device, the latter is nearly infinitely easier on the battery.

Now admittedly, if you had a very poor implementation of std::condition_variable, it could have the same inefficiency as the polling loop. However in practice such a vendor ought to go out of business fairly quickly.

Update

For grins I augmented my condition_variable wait loop with a spurious wakeup detector. I ran it again, and it did not print out anything. Not one spurious wakeup. That is of course not guaranteed. But it does demonstrate what a quality implementation can achieve.