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:

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

Answer 2

The method employed by your compiler installation of boost is supported for your target, whereas your installation of boost compiler does not support this feature for your target.

TFM says:

The number of hardware threads available on the current system (e.g. number of CPUs or cores or hyperthreading units), or 0 if this information is not available.

EDIT: scratch that, reverse it.

EDIT2: This feature is present on the trunk, but absent in 4.6.2:

~/tmp/gcc-4.6.2/libstdc++-v3/src> wc -l thread.cc
104 thread.cc
~/tmp/gcc-4.6.2/libstdc++-v3/src> grep concurrency thread.cc | wc -l
0
~/tmp/gcc-4.6.2/libstdc++-v3> grep -C 2 VERIFY testsuite/30_threads/thread/members/hardware_concurrency.cc

  // Current implementation punts on this.
  VERIFY( std::thread::hardware_concurrency() == 0 );

  return 0;