在C ++中自动生成代码

Question

I want a piece of code which does not involve loops but automatically generates some C++ code.

I have an const int d , and from this I want to write d lines of code to access an array. So for instance

for(int k=0; k<d;++k){
  // do something to myarryay[k];
}

but I don't want to write this in a for loop. I want the complier to execute as if the following lines of code was written:

do something to myarray[0]
do something to myarray[1]
.
.
.
do something to myarray[d]

Can anyone give me a suggestion on some code that does this?

Thanks in advance.

Answer 1

Are you sure you need to do this manually? This is an optimization known as loop unrolling . At high enough optimization levels, your compiler will do it for you, and possibly better than you can, since a good optimizing compiler will take into account the tradeoffs (reduced instruction cache locality, for one).

Answer 2

You should rarely need to manually unroll a loop (I'd say never, but if you're working on something with crazy performance requirements and you think you can one-up the compiler optimizer, then perhaps you might manually unroll a loop).

If you do need to do this for some reason, it's quite straightforward with the help of the preprocessor:

#include <boost/preprocessor.hpp>

#include <iostream>

void f(int x) { std::cout << x << std::endl; }

int main()
{
    #define MYARRAY_COUNT 10
    int myarray[MYARRAY_COUNT] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    #define GENERATE_ELEMENT_CASE(z, n, data) f(myarray[n]);

    BOOST_PP_REPEAT(MYARRAY_COUNT, GENERATE_ELEMENT_CASE, x)

    #undef GENERATE_ELEMENT_CASE
    #undef MYARRAY_COUNT
}

The expanded main() function looks like:

int main()
{
    int myarray[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    f(myarray[0]); f(myarray[1]); f(myarray[2]); f(myarray[3]); f(myarray[4]);
    f(myarray[5]); f(myarray[6]); f(myarray[7]); f(myarray[8]); f(myarray[9]);
}