Handle std::thread::hardware_concurrency() - c++

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 local repository of code or something), that this feature is not implemented for versions of g++ prior to 4.8.
As a matter of fact, I was the at the same victim position as this user. The function will simply return 0. I found in this answer a user implementation. Comments on whether this answer is good or not are welcome!
So I would like to do this in my code:
unsinged int cores_n;
#if g++ version < 4.8
cores_n = my_hardware_concurrency();
#else
cores_n = std::thread::hardware_concurrency();
#endif
However, I could find a way to achieve this result. What should I do?

Answer 1

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"));
}

Answer 2

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