C++ running timer in the background? - c++

Question

I've recently tried cpp, in the thing I'm making I'm trying to make it so that a variable with the value of 20 is subtracted by 1 every second, but I also need the machine to be waiting for an input from the user. I tried using for loops but they won't proceed until the input is placed or until the variable runs out. I looked at clock but they don't seem to fit my need, or maybe I just misunderstood their purpose.
Any suggestions?

Answer 1

As has already been suggested in the comments, threading is one way to do this. There is a nice self-contained example here (which I've borrowed from in the code below).
In the code below an asynchronous function is launched. Details on these here. This returns a future object which will contain the result once the job has finished.
In this case the job is listening to cin (typically the terminal input) and will return when some data is entered (i.e. when enter is pressed).
In the meantime the while loop will be running which keeps a track of how much time has passed, decrements the counter, and also returns if the asynchronous job finishes. It wasn't clear from your question if this is exactly the behaviour you want but it gives you the idea. It will print out value of decremented variable, but user can enter text, and it will print that out once user presses enter.
#include <iostream>
#include <thread>
#include <future>
#include <time.h>
int main() {
// Enable standard literals as 2s and ""s.
using namespace std::literals;
// Execute lambda asyncronously (waiting for user input)
auto f = std::async(std::launch::async, [] {
auto s = ""s;
if (std::cin >> s) return s;
});
// Continue execution in main thread, run countdown and timer:
int countdown = 20;
int countdownPrev = 0;
std::chrono::steady_clock::time_point begin = std::chrono::steady_clock::now();
std::chrono::steady_clock::time_point end;
double elapsed;
while((f.wait_for(5ms) != std::future_status::ready) && countdown >= 0) {
end = std::chrono::steady_clock::now();
elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(end - begin).count();
countdown = 20 - (int) (elapsed/1000);
if (countdown != countdownPrev) {
std::cout << "Counter now: " << std::fixed << countdown << std::endl;
countdownPrev = countdown;
}
}
if (countdown == -1) {
std::cout << "Countdown elapsed" << std::endl;
return -1;
} else {
std::cout << "Input was: " << f.get() << std::endl;
return 0;
}
}
P.S. to get this to work on my compiler I have to compile it with g++ -pthread -std=c++14 file_name.cpp to correctly link the threading library and allow use of c++14 features.