Handle std::thread::hardware_concurrency()

Question

In my question about std::thread, I was advised to use std::thread::hardware_concurrency(). I read somewhere (which I can not find it and seems like a

Answer 1

Based on common predefined macros link, kindly provided by Joachim, I did:

int p;
#if __GNUC__ >= 5 || __GNUC_MINOR__ >= 8 // 4.8 for example
  const int P = std::thread::hardware_concurrency();
  p = (trees_no < P) ? trees_no : P;
  std::cout << P << " concurrent threads are supported.\n";
#else
  const int P = my_hardware_concurrency();
  p = (trees_no < P) ? trees_no : P;
  std::cout << P << " concurrent threads are supported.\n";
#endif

Answer 2

There is another way than using the GCC Common Predefined Macros: Check if std::thread::hardware_concurrency() returns zero meaning the feature is not (yet) implemented.

unsigned int hardware_concurrency()
{
    unsigned int cores = std::thread::hardware_concurrency();
    return cores ? cores : my_hardware_concurrency();
}

You may be inspired by awgn's source code (GPL v2 licensed) to implement my_hardware_concurrency()

auto my_hardware_concurrency()
{
    std::ifstream cpuinfo("/proc/cpuinfo");

    return std::count(std::istream_iterator<std::string>(cpuinfo),
                      std::istream_iterator<std::string>(),
                      std::string("processor"));
}