Template Metaprogramming
In this lesson, we'll learn about template metaprogramming.
Template Metaprogramming
How this all started:
- 1994 Erwin Unruh discovered template metaprogramming by accident.
- His program failed to compile but calculated the first 30 prime numbers at compile-time.
- To prove his point, he used the error messages to display the first 30 prime numbers.
Let’s have a look at the screenshot of the error:
We have highlighted the important parts in red. We hope you can see the pattern. The program calculates at compile-time the first 30 prime numbers. This means template instantiation can be used to do math at compile-time. It gets even better. Template metaprogramming is Turing-complete and can, therefore, be used to solve any computational problem. Of course, Turing-completeness holds only in theory for template metaprogramming because the recursion depth (at least 1024 with C++11) and the length of the names which are generated during template instantiation provide some limitations.
Calculating at Compile-Time
The Factorial program is the Hello World of template metaprogramming.
template <int N>struct Factorial{static int const value= N * Factorial<N-1>::value;};template <> struct Factorial<1>{static int const value = 1;};std::cout << Factorial<5>::value << std::endl;std::cout << 120 << std::endl;
The call Factorial<5>::value in ...