Why is there a difference using std::thread::hardware_concurrency() and boost::thread::hardware_concurrency()?

Question

The description of the problem itself is pretty simple. I'm testing the differences of std::thread library in C++11 and boost::thread library.

The output of these:

#include <iostream> #include <thread> #include <boost/thread.hpp>  int main() {   std::cout << std::thread::hardware_concurrency() << std::endl;   std::cout << boost::thread::hardware_concurrency() << std::endl;   return 0; } 

gives me different results:

0 4 

Why is that?

PS: The version of the gcc package is 4.6.2-1.fc16 (x86_64). I'm using

g++ test.cc -Wall -std=c++0x -lboost_thread-mt -lpthread 

Answer 1

After reviewing /usr/include/c++/4.6.2/thread

it can be seen that the implementation is actually:

// Returns a value that hints at the number of hardware thread contexts. static unsigned int hardware_concurrency() { return 0; } 

So problem solved. It's just another feature that hasn't been implemented in gcc 4.6.2