calculating factorial using template meta-programming

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  • What is this weird template that takes <int N>?

In C++, template arguments can either be types (prefixed with class or typename) or integers (prefixed with int or unsigned int). Here we are in the second case.

  • What is this second weird template <>?

template<> struct Factorial<0> is a complete specialization of Factorial class template, which means that 0 is considered a special value to which corresponds its own version of Factorial.

  • What are the enums for?

enums are the way to compute values in metaprogramming C++

  • What is the advantage of using this rather than normal runtime factorial calculation?

The reason why this code was created in the first place is to create a proof of concept that calculus can be done using metaprogramming. The advantage is that generated code is extremely efficient (calling Factorial<4>::value is equivalent to simply writing “24” in your code.

  • How often do you people use this? I have been using C++ for a while now, but never used this before. How big a part of C++ was I missing out on?

Such functionality is rarely achieved using this method, but metaprogramming is used more and more nowadays. See Boost meta-programming library to get a hint of what can be done.

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