"Non-constant" constant can't be used as template argument

[Martin Magnusson]

It seems that the following code is not valid, but I need to find a
workaround. Both gcc and Comeau tells me that j can't be used as a
template argument, even though it's declared as a const unsigned. Is
there a way to get around this?

template< unsigned M >
class Tree
{
public:
Tree()
{
for (unsigned i = 0; i < M; ++i)
{
const unsigned j = i;
Compare<j> c; // j cannot be used as template argument
}
}

private:
template< unsigned N >
class Compare
{
public:
bool operator()( int i1, int i2 )
{ return i1 < i2; }
};
};

int main()
{
Tree<3> t;
return 0;
}

[JKop]

Martin Magnusson posted:

It seems that the following code is not valid, but I need to find a
workaround. Both gcc and Comeau tells me that j can't be used as a
template argument, even though it's declared as a const unsigned. Is
there a way to get around this?

template< unsigned M >
class Tree
{
public:
Tree()
{
for (unsigned i = 0; i < M; ++i)
{
const unsigned j = i;
Compare<j> c; // j cannot be used as template argument
}
}

private:
template< unsigned N >
class Compare
{
public:
bool operator()( int i1, int i2 )
{ return i1 < i2; }
};
};

int main()
{
Tree<3> t;
return 0;
}

j is not a compile time constant.

-JKop

[John Harrison]

"Martin Magnusson" <[e-mail]> wrote in message
news:1097221267 .vPROieUgxzGXwA H8QTqyrg@terane ws...

It seems that the following code is not valid, but I need to find a
workaround. Both gcc and Comeau tells me that j can't be used as a
template argument, even though it's declared as a const unsigned. Is
there a way to get around this?

The way round it is to use loop unrolling.

template< unsigned M >
class Tree
{
public:
Tree()
{
UnrolledLoop<0, M>::execute();
}
};

template< unsigned I, unsigned M >
struct UnrolledLoop
{
static void execute()
{
Compare<I> c;
UnrolledLoop<I + 1, M>::execute();
}
};
template< unsigned I >
struct UnrolledLoop<I, I>
{
static void execute()
{
}
};

This is completely untested but hopefully you get the idea.

john