问C++自动检测模板参数？

EN

template <typename T1, typename T2>
class Farm
{
  private:
    T1 *task1;
    T2 *task2;
  public:
    // save them so that I can use them when invoking call operator
    Farm(T1 *_t1, T2 *_t2): task1(_t1), task2(_t2) { }

    void operator()()
    {
      // invoke call operator, meaning a farm could be a task (arbitrary nesting)
      (*task1)();
      (*task2)();
    }
};
int main()
{
    ... create two pointer(A *a, B *b...)
    Farm(a,b); // error: missing template arguments before ‘(’ token

    Farm<A, B>(a,b); // in this works, it works
}