The parameter grainsize must be positive. The debug version of the library raises an assertion failure if this requirement is not met.
Constructs a blocked_range representing the half-open interval [ begin, end) with the given grainsize.
The statement " blocked_range<int> r( 5, 14, 2 );" constructs a range of int that contains the values 5 through 13 inclusive, with a grainsize of 2. Afterwards, r.begin()==5 and r.end()==14.