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#include <iostream>
 
template<typename T>
class Vector2
{
public:
  Vector2(T x, T y)
  : x(x), y(y)
  {}
 
  template<typename U>
  Vector2<U> cast() const { return Vector2<U>((U)x, (U)y); }
 
  T x;
  T y;
};
 
template<typename T>
class Line2
{
public:
  Line2(Vector2<T> a, Vector2<T> b)
  : a(a), b(b)
  {}
 
  double gradient() const
  {
    Vector2<double> ad(a.cast<double>());
    Vector2<double> bd(b.cast<double>());
 
    return (bd.y - ad.y) / (bd.x - ad.x);
  }
 
  Vector2<T> a;
  Vector2<T> b;
};
 
int main()
{
  Vector2<int> intVector(10,5);
  Vector2<double> doubleVector(intVector.cast<double>());
 
  Line2<int> intLine(Vector2<int>(0,0), Vector2<int>(1,3));
  Line2<double> doubleLine(Vector2<double>(0,0), Vector2<double>(0.5,1.23));
 
  std::cout << "Line2<int>.gradient = " << intLine.gradient() << std::endl;
  std::cout << "Line2<double>.gradient = " << doubleLine.gradient() << std::endl;
}
 

prog.cpp: In member function ‘double Line2<T>::gradient() const’:
prog.cpp:28:31: error: expected primary-expression before ‘double’
     Vector2<double> ad(a.cast<double>());
                               ^
prog.cpp:28:31: error: expected ‘)’ before ‘double’
prog.cpp:29:31: error: expected primary-expression before ‘double’
     Vector2<double> bd(b.cast<double>());
                               ^
prog.cpp:29:31: error: expected ‘)’ before ‘double’

Standard output is empty