Calculating the area of a circle
I assume one of the complaints was with the constant.  There are various ways to calculate pi, using an exact calculation rather than using some number of digits.

Here's one example (in C++14):

constexpr auto pi()
{
    return std::acos(-1);
}


(For C++11, use a non-auto type, such as double or float.)

The constexpr keyword will allow this to be calculated at compile time.  You could still assign the return value to a constant or call the function inline where needed.  If you don't have C++11, then you can use const in place of constexpr, and just assign it to a constant.