CSC/ECE 517 Spring 2013/ch1 1h jc

Metaprogramming in statically typed languages

Common Uses

Pre-generate static data at compile time

One common use of metaprogramming in statically typed languages is to write programs that will pre-generate tables of data for use at runtime.

A practicable approach to pre-generate static data at compile time in C++ is template metaprogramming. Template metaprograms consist of class templates operating on numbers and/or types as data. Algorithms are expressed using template recursion as a looping construct and class template specialization as a conditional construct. Template recursion involves the direct or indirect use of a class template in the construction of its own member type or member constant.

Here is an example of a class template which computes the factorial of a natural number:

        template<int n>
        struct Factorial
        {
           enum { RET = Factorial<n-1>::RET * n };
        };
        //the following template specialization terminates the recursion
        template<>
        struct Factorial<0>
        {
          enum { RET = 1 };
        };

We can use this class template as follows:

        void main()
        {
          cout << Factorial<7>::RET << endl; //prints 5040
        }

The important point about this program is that Factorial<7> is instantiated at compile time. During the instantiation, the compiler also determines the value of Factorial<7>::RET. Thus, the code generated for this main() program by the C++ compiler is the same as the code generated for the following main():

        void main()
        {
        cout << 5040 << endl; //prints 5040
        }