Getting template metaprogramming compile-time constants at runtime
- by GMan - Save the Unicorns
Background
Consider the following:
template <unsigned N>
struct Fibonacci
{
enum
{
value = Fibonacci<N-1>::value + Fibonacci<N-2>::value
};
};
template <>
struct Fibonacci<1>
{
enum
{
value = 1
};
};
template <>
struct Fibonacci<0>
{
enum
{
value = 0
};
};
This is a common example and we can get the value of a Fibonacci number as a compile-time constant:
int main(void)
{
std::cout << "Fibonacci(15) = ";
std::cout << Fibonacci<15>::value;
std::cout << std::endl;
}
But you obviously cannot get the value at runtime:
int main(void)
{
std::srand(static_cast<unsigned>(std::time(0)));
// ensure the table exists up to a certain size
// (even though the rest of the code won't work)
static const unsigned fibbMax = 20;
Fibonacci<fibbMax>::value;
// get index into sequence
unsigned fibb = std::rand() % fibbMax;
std::cout << "Fibonacci(" << fibb << ") = ";
std::cout << Fibonacci<fibb>::value;
std::cout << std::endl;
}
Because fibb is not a compile-time constant.
Question
So my question is:
What is the best way to peek into this table at run-time? The most obvious solution (and "solution" should be taken lightly), is to have a large switch statement:
unsigned fibonacci(unsigned index)
{
switch (index)
{
case 0:
return Fibonacci<0>::value;
case 1:
return Fibonacci<1>::value;
case 2:
return Fibonacci<2>::value;
.
.
.
case 20:
return Fibonacci<20>::value;
default:
return fibonacci(index - 1) + fibonacci(index - 2);
}
}
int main(void)
{
std::srand(static_cast<unsigned>(std::time(0)));
static const unsigned fibbMax = 20;
// get index into sequence
unsigned fibb = std::rand() % fibbMax;
std::cout << "Fibonacci(" << fibb << ") = ";
std::cout << fibonacci(fibb);
std::cout << std::endl;
}
But now the size of the table is very hard coded and it wouldn't be easy to expand it to say, 40.
The only one I came up with that has a similiar method of query is this:
template <int TableSize = 40>
class FibonacciTable
{
public:
enum
{
max = TableSize
};
static unsigned get(unsigned index)
{
if (index == TableSize)
{
return Fibonacci<TableSize>::value;
}
else
{
// too far, pass downwards
return FibonacciTable<TableSize - 1>::get(index);
}
}
};
template <>
class FibonacciTable<0>
{
public:
enum
{
max = 0
};
static unsigned get(unsigned)
{
// doesn't matter, no where else to go.
// must be 0, or the original value was
// not in table
return 0;
}
};
int main(void)
{
std::srand(static_cast<unsigned>(std::time(0)));
// get index into sequence
unsigned fibb = std::rand() % FibonacciTable<>::max;
std::cout << "Fibonacci(" << fibb << ") = ";
std::cout << FibonacciTable<>::get(fibb);
std::cout << std::endl;
}
Which seems to work great. The only two problems I see are:
Potentially large call stack, since calculating Fibonacci<2 requires we go through TableMax all the way to 2, and:
If the value is outside of the table, it returns zero as opposed to calculating it.
So is there something I am missing? It seems there should be a better way to pick out these values at runtime.
A template metaprogramming version of a switch statement perhaps, that generates a switch statement up to a certain number?
Thanks in advance.
