Actual millis in a C++

Question

Is it possible to get the actual millis since I-don't-know in a C++-programm like System.currentTimeMillis() in Java? I know time(), but I think it's not exactly enough to measure short times, is it?

Solution 1

It's part of the language standard these days (some years now):

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#include <chrono>
#include <iostream>

int main()
{
    using namespace std::chrono;

    auto epoch = high_resolution_clock::from_time_t(0);
    // ...
    auto now   = high_resolution_clock::now();

    auto mseconds = duration_cast<milliseconds>(now - epoch).count();

    std::cout << "millis: " << mseconds;
}

Solution 2

In C++ you also have clock()

#include <time.h>

...

clock_t start = clock();
... some processing ...
clock_t stop = clock();

double elapsed = double(stop - start) / CLOCKS_PER_SEC;

There are also other more accurate ways of measure timings, but they are more dependent on which king of system you're running your programs on.

Solution 3

If you have C++11 support available, you may want to look into std::chrono.

#include <iostream>
#include <chrono>
#include <ctime>

long fibonacci(int n)
{
    if (n < 3) return 1;
    return fibonacci(n-1) + fibonacci(n-2);
}

int main()
{
    std::chrono::time_point<std::chrono::system_clock> start, end;
    start = std::chrono::system_clock::now();
    std::cout << "f(42) = " << fibonacci(42) << '\n';
    end = std::chrono::system_clock::now();

    std::chrono::duration<double> elapsed_seconds = end-start;
    std::time_t end_time = std::chrono::system_clock::to_time_t(end);

    std::cout << "finished computation at " << std::ctime(&end_time)
              << "elapsed time: " << elapsed_seconds.count() << "s\n";
}

Failing that, you could use the C style std::time as follows:

#include <ctime>
#include <iostream>

int main()
{
    std::time_t result = std::time(NULL);
    std::cout << std::asctime(std::localtime(&result))
              << result << " seconds since the Epoch\n";
}

Comments

[manlio]

Take a look at these questions: how to get time in millis in c++ just like java and C++ Timer function to provide time in nano seconds

[László Papp]

@slugonamission: that is *nix specific...

[slugonamission]

@LaszloPapp - oops, deleted.

[PEAR]

Ok, is there any constant to get milliseconds?

[smoothware]

+1 This is how I would do it in C++. Note: using namespace std::chrono; seems very worth it here.

[David Schwartz]

The clock function returns CPU time. I get the impression this is asking about wall time.

[sehe]

What do seconds sinds the epoch have to do with things?

[László Papp]

@sehe: read about System.currentTimeMillis in the original question, please. This may be a good starting point.

[sehe]

Changed the sample to actually print milliseconds since the epoch

[sehe]

Although even MSVC, the compiler-that-always-lags-behing, had <chrono> in VS2012

[6502]

@pear: what about multiplying the result of the division by 1000? :-)

[Walter]

@LaszloPapp This will not work without C++11 support -- there no point to consider outdated standards, unless you're dealing with legacy code.

[László Papp]

@Walter: clearly, you have not worked much with embedded. I understand bleeding edge desktop people hardly see out of their box, but that is just a specific domain. To be fair, even on desktop there are plenty of VS2010 and VS2008 users, not to mention Windows CE, etc.

[sehe]

Okay, time to calm down people. It's all useful information. No need to imply incompetence left or right

[Matthieu M.]

@LaszloPapp: And let's not forget corporate world, the migration from gcc 3.4 to gcc 4.3 is still ongoing where I work (has been for the last 7 years)...

[László Papp]

@MatthieuM.: yes, I agree.

[PEAR]

@6502: Of course, but this would not improve the accuracy, would it?

[6502]

@PEAR: the value of the constant CLOCKS_PER_SEC is system dependent and the same goes for the provided accuracy (they are not required to be aligned; e.g. CLOCKS_PER_SEC could be 100000 but the accuracy just 20ms).

[Tad]

should the code say auto epoch = system_clock::from_time_t(0); ? My code wouldn't compile with high_resolution_clock::from_time_t(0);

[sehe]

@Tad your compiler might not be to update/have full support for std::chrono (yet): en.cppreference.com/w/cpp/chrono/high_resolution_clock. Depending on the resolution you desire, system_clock could be fine for your purposes

[Tad]

I used this code and time calculated was 1412002099144 miliseconds. Is time 0 in 1970? Why not use system_clock::now()? I am using gcc 4.9

[sehe]

@Tad look at the question for context. Heck, here's a link for you docs.oracle.com/javase/7/docs/api/java/lang/…