Explicit Template Instantiation

[balajiram]

Please refer the article: http://www.cplusplus.com/forum/articles/14272/

I am using the method described in the article for explicit template instantiation in a separate .cpp file with my class declaration in a .h file. Here is the description of the class:

template <int N>
class vector {
public:
	double basis[N];
public:
	vector<N>(){for(int i=0;i<N;i++)basis[i]=0;}
	vector<N>(double values[N]) {for(int i=0;i<N;i++)basis[i]=values[i];}
	vector<N> operator= (vector<N>);
	vector<N> operator+ (vector<N>);
	double operator* (vector<N>);
	vector<N> operator* (double);
	vector<N> operator- (vector<N>);
//	vector operator^ (vector);		// Not needed right now
	double mag ();
	vector<N> operator+= (vector<N>);
	vector<N> operator-= (vector<N>);
	vector<N> operator*= (double);
	bool operator== (vector<N>);
	template <unsigned int M>
		friend vector<M> operator* (double, vector<M>);
	vector<N> operator/ (double);
};

The .cpp file contains all definitions of these functions. In particular I have the friend function also defined:

template <int N>
vector<N> operator* (double l, vector<N> a) {
	vector<N> r;
	for (int i=0;i<N;i++)
		r.basis[i]=a.basis[i]*l;
	return r;
}

In the end, I am explicitly instantiating:

template class vector<1>;
template class vector<2>;
template class vector<3>;

However, still, the compiler complains that there is no definition for the function 'operator*' that takes (double, vector<1>) or (double, vector<2>) or (double, vector<3>) as arguments. I need this feature to be present. Any help will be appreciated.

[kbw]

You need to move that function definition to the header file and make it inline.

Template functions are generated on the fly. If the definition of a methid isn't seen, the compiler doesn't generate code for it. Templates, at first glance, appear to be different in this regard.

[balajiram]

I have tried it. Here is what I did:

template <int N>
class vector {
public:
	double basis[N];
public:
	vector<N>(){for(int i=0;i<N;i++)basis[i]=0;}
	vector<N>(double values[N]) {for(int i=0;i<N;i++)basis[i]=values[i];}
	vector<N> operator= (vector<N>);
	vector<N> operator+ (vector<N>);
	double operator* (vector<N>);
	vector<N> operator* (double);
	vector<N> operator- (vector<N>);
//	vector operator^ (vector);		// Not needed right now
	double mag ();
	vector<N> operator+= (vector<N>);
	vector<N> operator-= (vector<N>);
	vector<N> operator*= (double);
	bool operator== (vector<N>);
	template <unsigned int M>
		friend vector<M> operator* (double l, vector<M> a) {
			vector<M> r;
			for (int i=0;i<M;i++)
				r.basis[i]=a.basis[i]*l;
			return r;
		}
	vector<N> operator/ (double);
};

But it's still not working for me. In one place I have the following code:

template <int N>
void atom<N>::move(double tstep, vector<N> g, vector<N> Ef, vector<N> Bf) {
	particle<N>::k+=g*(mass*tstep/GSL_CONST_MKSA_PLANCKS_CONSTANT_HBAR);
	vector<N> delv = groupv(particle<N>::k) - particle<N>::v;
	vector<N> a = delv/tstep;
	particle<N>::r += (particle<N>::v*tstep + 0.5*a*(tstep*tstep));
	particle<N>::updateEv();
	for(int i=0;i<N;i++)
		Ef.basis[i]=Bf.basis[i]=0;
}


template class atom<1>;
template class atom<2>;
template class atom<3>;

In the above code, the line where I have g*(mass*tstep/GSL_CONST_MKSA_PLANCKS_CONSTANT_HBAR) works fine, but the line with 0.5*a*(tstep*tstep)) doesn't work. This is the functionality I am trying to achieve using the friend function. Of course, I could cut the crap and simply divide by 2, which is what I do to let it compile and let me work go on, but I wonder what happened to the function I wrote and would like to have all features enabled.

Thanks for helping.

[balajiram]

Ok I got it. Sorry. Here is what I had to finally do:

template <int N>
class vector {
public:
	double basis[N];
public:
	vector<N>(){for(int i=0;i<N;i++)basis[i]=0;}
	vector<N>(double values[N]) {for(int i=0;i<N;i++)basis[i]=values[i];}
	vector<N> operator= (vector<N>);
	vector<N> operator+ (vector<N>);
	double operator* (vector<N>);
	vector<N> operator* (double);
	vector<N> operator- (vector<N>);
//	vector operator^ (vector);		// Not needed right now
	double mag ();
	vector<N> operator+= (vector<N>);
	vector<N> operator-= (vector<N>);
	vector<N> operator*= (double);
	bool operator== (vector<N>);
	friend vector<N> operator* (double l, vector<N> a) {
		vector<N> r;
		for (int i=0;i<N;i++)
			r.basis[i]=a.basis[i]*l;
		return r;
	}
	vector<N> operator/ (double);
};

Thanks a lot for the help. Worked like a charm.