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template function w/ type & non-type parameters

Posted on 2008-06-24
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Last Modified: 2013-12-14
I have the following class definitions:

template<unsigned dim>
class Vector
{ ... };

template<unsigned dim>
class Displacement : public Vector<dim>
{ ... };
template<unsigned dim>
class Velocity : public Vector<dim>
{ ... };
template<unsigned dim>
class Acceleration : public Vector<dim>
{ ... };

etc.

Now I want to overload binary operators +, - , *, /. But I don't want to write member functions for all vector types. (there are too many.) So I wrote the following template function:
      template<typename T, unsigned dim>
      T<dim> operator + (const T<dim> & t0, const T<dim> & t1)                 <<<< C2988 on this line
      {
            T<dim> ret;
            for (int i=0; i<dim; i++)
                  ret[i] = t0[i] + t1[i];
      }
But I get compiler error C2988 (unrecognizable template declaration/definition; Visual Studio Pro 2008). The help page (http://msdn.microsoft.com/en-us/library/f69a8fb8.aspx) is pretty useless.
Is it possible to write template like above (of course, w/o syntax errors) or do I have no choice but to copy-paste for all types derived from Vector<dim>?
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Question by:jhshukla
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Expert Comment

by:jkr
ID: 21862056
Hmmm, I guess you meant the following:
      template<typename T, unsigned dim>
      T operator + (const T & t0[dim], const T & t1[dim])
      {
            T ret[dim];
            for (int i=0; i<dim; i++)
                  ret[i] = t0[i] + t1[i];
      }

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Author Comment

by:jhshukla
ID: 21862464
no I did not mean that.
The code you suggested would create (well, it is reference. so technically not "create" but let me use that term here anyways) an array of T's. I want one variable of type T<dim>.

and if [] inside the function confused you, it is using overloaded [] operator. to be more elaborate this is how Vector class is defined:
	template<unsigned dim>
	class Vector
	{
	public:
		Vector(double *init = NULL)
		{
			if (init == NULL)
				memset(components, 0, sizeof(components));
			else
				memcpy(components, init, sizeof(components));
		}
		~Vector(void) {}
 
		inline double operator[](int i) const { return components[i]; }
		inline double & operator[](int i) { return components[i]; }
 
/*
		these commented out functions are the ones I want to mimic using
		templates
		inline Vector<dim> operator/(double div) const
		{
			Vector<dim> ret;
			for (int i=0; i<dim; i++)
				ret[i] = components[i] / div;
		}
		inline Vector<dim> operator*(double mul) const
		{
			Vector<dim> ret;
			for (int i=0; i<dim; i++)
				ret[i] = components[i] * mul;
		}
 
		inline Vector<dim> operator+(Vector<dim> that) const
		{
			Vector<dim> ret;
			for (int i=0; i<dim; i++)
				ret[i] = components[i] + that[i];
		}
		inline Vector<dim> operator-(Vector<dim> that) const
		{
			Vector<dim> ret;
			for (int i=0; i<dim; i++)
				ret[i] = components[i] - that[i];
		}
*/
		inline double MagnitudeSq() const
		{ return dot(*this, *this); }
		inline double Magnitude() const
		{ return sqrt(MagnitudeSq()); }
		inline static double dot(const Vector<dim> & v0, const Vector<dim> & v1)
		{
			double accum = 0;
			for (int i=0; i<dim; i++)
				accum += v0[i]*v1[i];
			return accum;
		}
 
	private:
		double components[dim];
	};

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Author Comment

by:jhshukla
ID: 21862492
Forgot to mention that T will always be a derived class of Vector<dim>.

I expect to be able to use the template like this:
Displacement<3> d1, d2, d3;
d3 = d1 + d2;

The addition statement will cause the following function to be instantiated:
Displacement<3> operator + (const Displacement<3> &t0, const Displacement<3> &t1)
{ ... }

I found something called "template template parameter." no, it is not a typo. the word "template" is repeated. but cannot get it to work. e.g.
http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/topic/com.ibm.vacpp6m.doc/language/ref/clrc16template_template_parameters.htm
and with some explanation: http://www.informit.com/articles/article.aspx?p=376878

I modified the function template after reading InformIT article but I get even more errors now:
      template<template<unsigned dim> T>
      T operator + (const T & t0, const T & t1)
      {
            T ret;
            for (int i=0; i<dim; i++)
                  ret[i] = t0[i] + t1[i];
      }
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LVL 9

Author Comment

by:jhshukla
ID: 21862571
update. I made a mistake above. it is supposed to be the following:
      template<template<unsigned dim> class T>
      T operator + (const T & t0, const T & t1)                   <<< C3205 here
      {
            T ret;
            for (int i=0; i<dim; i++)
                  ret[i] = t0[i] + t1[i];
      }

however, now I am getting the error: "C3205: argument list for template template parameter 'T' is missing."
If I replace it with T<dim> it says "error C2065: 'dim' : undeclared identifier."
      template<template<unsigned dim> class T>
      T<dim> operator + (const T<dim> & t0, const T<dim> & t1)
      {
            T<dim> ret;
            for (int i=0; i<dim; i++)
                  ret[i] = t0[i] + t1[i];
      }
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LVL 9

Accepted Solution

by:
jhshukla earned 0 total points
ID: 21862696
phew. finally figured it out.

found clue here: http://hal.iwr.uni-heidelberg.de/lehre/prosem-06/Popov.pdf
scroll to p11

// This works:
	template<template<unsigned> class T, unsigned dim>
	T<dim> operator + (const T<dim> & t0, const T<dim> & t1)
	{
		T<dim> ret;
		for (int i=0; i<dim; i++)
			ret[i] = t0[i] + t1[i];
		return ret;
	}
 
// This should work too (template parameters are switched. nothing big):
	template<unsigned dim, template<unsigned> class T>
	T<dim> operator + (const T<dim> & t0, const T<dim> & t1)
	{
		T<dim> ret;
		for (int i=0; i<dim; i++)
			ret[i] = t0[i] + t1[i];
		return ret;
	}
 
// after this we can use statements like:
Displacement<3> d1, d2, d3;
d3 = d1 + d2;
Velocity<2> v1, v2, v3;
v1 = v2 + v3;

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