nvalid use of member function ‘int benchmark::State::threads() const’ (did you forget the ‘()’ ?)

最近用cmake编译一个c++程序，linux下编译的时候报错：invalid use of member function ‘int benchmark::State::threads() const’ (did you forget the ‘()’ ?)，这是非成员函数应该在函数外定义然后引进来吗？

报错内容和报错那一行的代码发过来