C++ for Scientists - Technische Universität Dresden

134 CHAPTER 5. META-PROGRAMMING
expression during a C ++ compilation [?]. Todd Veldhuizen showed that the template type
system of C ++ is Turing complete [?].
On the other hand, excessive usage of meta-programming techniques can end in quite long
compile times. Entire research projects were cancelled after many millions dollars of funding
because even short applications of less than 20 lines took weeks of compile time on parallel
computers. We know people who managed to produce a 18 MB error message (it came mainly
from one single error). Nevertheless, the authors used a fair amount of meta-programming in
their scientific projects and could still avoid exhaustive compile times. 1 Also compilers improved
significantly in the last decade. Meanwhile the compile time grew quadratically in the template
instantiation depth in old compilers, today compile grows only linearly [?].
5.1 Let the Compiler Compute
Typical introduction examples for meta-programming are factorial and Fibonacci numbers. It
is computed recursively:
template
struct fibonacci
{
static const long value= fibonacci::value + fibonacci::value;
};
template
struct fibonacci
{
static const long value= 1;
};
template
struct fibonacci
{
static const long value= 1;
};
Note that we need the specialization for 1 and 2 to terminate the recursion. The following
definition:
template
struct fibonacci
{
static const long value= N < 3 ? 1 : fibonacci::value + fibonacci::value; // error
};
ends in an infinite compile loop. For N = 2, the compiler would evaluate the expression:
template
struct fibonacci
{
static const long value= 2 < 3 ? 1 : fibonacci::value + fibonacci::value; // error
};
1 TODO: Oder René?