Why does coliru return the same values for chrono::system_clock::now().time_since_epoch()?

Question

While testing someone else's code on coliru, I noticed that std::chrono::system_clock::now().time_since_epoch() returns the same values on multiple runs. I tested the following code with g++ and clang++ on coliru.

#include <iostream>
#include <chrono>
int main()
{
    auto seed = std::chrono::system_clock::now().time_since_epoch().count();
    std::cout << seed << "\n";
    std::cout << std::chrono::system_clock::now().time_since_epoch().count() << "\n";

    return 0;
}

Output:

g++ -std=c++11 -O2 -Wall -pedantic -pthread main.cpp && ./a.out

1433249917642594133

1433249917642674289

It does work as expected on ideone and obviously on my computer.

Does anyone know why coliru returns the same values on every run?

if you edit the code slightly in each run e.g. add white-space then results vary in each run — 911, Jun 02 '15 at 13:11
@911 Ok, interesting. Weird. So the implementation has nothing to do with the `system_clock`. My question still stands. Or it's even broader. Is colirus version of `chrono` not reliable? — Avi Ginsburg, Jun 02 '15 at 13:17
@911 Got it. I opened a private window in Firefox, copy & pasted the code and got a different number. However, if you add whitespace on run number 2, and remove the same whitespace on run number 3, the result is the same as run number 1. — Avi Ginsburg, Jun 02 '15 at 13:27
coliru is probably caching run results so if you rerun the same code it will serve up the last ran data. — NathanOliver, Jun 02 '15 at 13:35

score 1 · Accepted Answer · answered Jun 02 '15 at 14:02

1

Coliru caches the results of each snippet, so this is expected behaviour.

You can force a re-run by trivially altering the source file (by, say, adding more whitespace, or changing the contents of a comment).

_{(Source: I know the author.)}

answered Jun 02 '15 at 14:02

Lightness Races in Orbit

378,754
76
643
1,055

1

Ok, I can't argue with that source. – Avi Ginsburg Jun 02 '15 at 14:06

Why does coliru return the same values for chrono::system_clock::now().time_since_epoch()?

1 Answers1