Java – How to drive C#, C++ or Java compiler to compute 1+2+3+…+1000 at compile time

ccompiler-constructionjavametaprogramming

In a recent interview, I was asked a really strange question. The interviewer asked me how can I compute 1+2+3+…+1000 just using compiler features. This means that I am not allowed to write a program and execute it, but I should just write a program that could drive the compiler to compute this sum while compilation and print the result when compilation completes. As a hint, he told me that I may use generics and pre-processor features of the compiler. It is possible to use C++, C# or Java compiler. Any ideas???

This question is not related to computing the sum without any loops asked here. In addition, It should be noted that the sum SHOULD be calculated during compilation. Printing just the result using C++ compiler directives is not acceptable.

Reading more about the posted answers, I found that solving problems during compilation using C++ templates is called metaprogramming. This is a technique that was discovered accidentally by Dr. Erwin Unruh, during the process of standardizing the C++ language. You may read more about this topic on wiki page of meta-programming.
It seems that it is possible to write the program in Java using java annotations. You may take a look at maress's answer below.

A nice book about meta-programming in C++ is this one. Worth to take a look if interested.

A useful C++ meta-programming library is Boost's MPL this link.

Best Answer

Updated Now with improved recursion depth! Works on MSVC10 and GCC without increased depth. :)

Simple compile-time recursion + addition:

template<unsigned Cur, unsigned Goal>
struct adder{
  static unsigned const sub_goal = (Cur + Goal) / 2;
  static unsigned const tmp = adder<Cur, sub_goal>::value;
  static unsigned const value = tmp + adder<sub_goal+1, Goal>::value;
};

template<unsigned Goal>
struct adder<Goal, Goal>{
  static unsigned const value = Goal;
};

Testcode:

template<unsigned Start>
struct sum_from{
  template<unsigned Goal>
  struct to{
    template<unsigned N>
    struct equals;

    typedef equals<adder<Start, Goal>::value> result;
  };
};

int main(){
  sum_from<1>::to<1000>::result();
}

Output for GCC:

error: declaration of ‘struct sum_from<1u>::to<1000u>::equals<500500u>’

Live example on Ideone.

Output for MSVC10:

error C2514: 'sum_from<Start>::to<Goal>::equals<Result>' : class has no constructors
      with
      [
          Start=1,
          Goal=1000,
          Result=500500
      ]
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