Static variable VS local variable

static_or_private.cpp

class FooA
{
public:
  FooA(){}
  
  double calculateAnswer(double val)
  {
    static double prev_val_ = 0;
    if( val == 0.0 ) return 0.0;
    
    if( prev_val_ != val)
    {
      prev_val_ = val;
      cached_val_ = longCalculation(val);
    }
    return cached_val_;
  }
private:
  double cached_val_;
}

class FooB
{
public:
  FooB():prev_val_(0.0) {}
  
  double calculateAnswer(double val)
  {
    if( val == 0.0 ) return 0.0;
    
    if(prev_val_ != val)
    {
      prev_val_ = val;
      cached_val_ = longCalculation(val);
    }
    return cached_val_;
  }
private:
  double cached_val_;
  double prev_val_ ;
}