Question

I have difficulties to understand how the following template definition and template specialization definition work? To me, factorial<34> or factorial<T-1> look strange!

For example:

factorial<T - 1>::value

means what?

#include <iostream>

template<int T>
struct factorial {
  enum { value = factorial<T - 1>::value * T };
};

template<>
struct factorial<1> {
  enum { value = 1 };
};

int main()
{
  std::cout << factorial<34>::value << std::endl;

}    

g++ -o testSTL01 testSTL01.cpp -Wall
testSTL01.cpp: In instantiation of ‘factorial<13>’:
testSTL01.cpp:5:3:   instantiated from ‘factorial<14>’
testSTL01.cpp:5:3:   instantiated from ‘factorial<15>’
testSTL01.cpp:5:3:   instantiated from ‘factorial<16>’
testSTL01.cpp:5:3:   instantiated from ‘factorial<17>’
testSTL01.cpp:5:3:   instantiated from ‘factorial<18>’
testSTL01.cpp:5:3:   [ skipping 11 instantiation contexts ]
testSTL01.cpp:5:3:   instantiated from ‘factorial<30>’
testSTL01.cpp:5:3:   instantiated from ‘factorial<31>’
testSTL01.cpp:5:3:   instantiated from ‘factorial<32>’
testSTL01.cpp:5:3:   instantiated from ‘factorial<33>’
testSTL01.cpp:5:3:   instantiated from ‘factorial<34>’
testSTL01.cpp:15:29:   instantiated from here
testSTL01.cpp:5:3: warning: integer overflow in expression
start to run the app ...

0

Answer 1

This is an example of template metaprogramming. This program calculates factorial at compile time using recursion. The base of recursion is here:

template<>
struct factorial<1> {
  enum { value = 1 };
};

It says that factorial of 1 is 1.

The other template simply says that factorial of a number is that number times factorial minus 1.

template<int T>
struct factorial {
  enum { value = factorial<T - 1>::value * T };
};

Since there is really no “calling” in the classic sense, the template instantiates itself with a template argument equal to T-1 calculated at compile time.

P.S. The warning shows that factorial of 34 overflows 32-bit integer.