Re: Template instantiation problem, take 2
Kaba <none@here.com> writes:
Hello, I posted this problem to comp.lang.c++, but since I didn't get a
satisfying answer, I decided to post here.
Here is a short snippet of code that does not compile (tested with Vc8
and Comeau). This is because somehow Vector<0> gets instantiated, for
which the array size goes to 0. However, I don't quite get it why it
gets instantiated in the first place. So why does not this compile?
template <int N>
struct Vector
{
int data_[N];
};
template <int HEIGHT, int WIDTH>
struct Matrix
{
};
template <int HEIGHT, int WIDTH>
Vector<WIDTH> operator *(
const Vector<HEIGHT>&,
const Matrix<HEIGHT, WIDTH>&)
{
return Vector<WIDTH>();
}
template <int HEIGHT, int WIDTH>
Vector<WIDTH - 1> operator *(
const Vector<HEIGHT - 1>&,
const Matrix<HEIGHT, WIDTH>&)
{
return Vector<WIDTH - 1>();
}
int main()
{
Matrix<1, 1> transform;
Vector<1> translation;
translation = translation * transform;
return 0;
}
The problem here is that even though the 2nd overload doesn't match,
in order to determine that it's a non-match it needs to instantiate
the argument types. The second overload is "more specialized" than
the first (see the partial ordering algorithm in the standard), so if
the compiler starts trying to match with the last argument, it gets
HEIGHT == 1 and WIDTH == 1. Then it needs to see whether the first
argument, a Vector<1> lvalue, matches Vector<0> const&. You might
think it obviously doesn't, but consider this definition of Vector:
template <int N>
struct Vector
{
Vector();
Vector(Vector<N+1> const&);
int data_[N > 0 ? N : 1];
};
In this case it would be a match. The upshot is that the compiler
needs to instantiate the entire definition of Vector<0> (including the
declarations of its members) in order to rule out the 2nd overload.
You can work around this issue by replacing the 2nd overload with:
template <int height_minus_1, int WIDTH>
Vector<WIDTH - 1> operator *(
const Vector<height_minus_1>&,
const Matrix<height_minus_1 + 1, WIDTH>&)
{
return Vector<WIDTH - 1>();
}
HTH,
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com
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