C++库研究笔记 error: templates may not be ‘virtual’

class function
{
public:
    template <typename T>
    virtual T operator()(const array1d<T>& x, array1d<T>& g)const=0;
    virtual ::std::string functionName()const { return "f(x)"; }
    virtual ::std::string gradientName()const { return "f'(x)"; }
};