Question

we’re taking some code written for Visual Studio 2008 and try to compile it with gcc. We experienced an error in the following code (simplified to what’s necessary):

template<int R, int C, typename T>
struct Vector
{
 template <typename TRes>
 TRes magnitude() const
 {
  return 0;
 }

};

struct A
{
 typedef Vector<3,1,int> NodeVector;
};

template<class T>
struct B
{
 void foo()
 {
  typename T::NodeVector x;
  x.magnitude<double>(); //< error here
 }
};

...
    B<A> test;
    test.foo();

GCC says

error: expected primary-expression before 'double'
error: expected `;' before 'double'

Can you explain the error to me? What’s a cross-compiler solution?

Thanks a lot!

Answer 1

The problem is that since the C++ compiler doesn’t know the actual type of T (let alone T::NodeVector it doesn’t know that magnitude is supposed to be a template. You need to specify that explicitly:

x.template magnitude<double>();

Otherwise C++ will parse the tokens as x, operator., magnitude, operator<, double, operator> …

The GCC is right, by the way. MSVC++ is notoriously lax on such matters.