C++ for Scientists - Technische Universität Dresden

Meta-programming
Chapter 5
‘Meta-programming’ is actually discovered by accident. Erwin Unruh wrote in the early 90’s
a program that printed prime number as error messages. This showed that C ++ compilers can
compute. Because the language has changed since Unruh wrote the example, here is a version
adapted to today’s standard C ++:
// Prime number computation by Erwin Unruh
template struct D { D(void∗); operator int(); };
template struct is prime {
enum { prim = (p==2) || (p%i) && is prime2?p:0), i−1> :: prim };
};
template struct Prime print {
Prime print a;
enum { prim = is prime::prim };
void f() { D d = prim ? 1 : 0; a.f();}
};
template struct is prime { enum {prim=1}; };
template struct is prime { enum {prim=1}; };
template struct Prime print {
enum {prim=0};
void f() { D d = prim ? 1 : 0; };
};
main() {
Prime print a;
a.f();
}
When tried to compile with g++ 4.1.2, one will observe the following error message: TODO:
Need English error message.
TODO: Ask Erwin Unruh if we can use his example.
After people realized the computational power of the C ++ compiler, it was used to realize very
powerful performance optimization techniques. In fact, one can perform entire applications
during compile time. Jeremiah Wilcock once wrote a Lisp interpreter that evaluated Lisp
133