To provide a concrete example, we imagine that we need to write a code to verify whether a number is prime. Many algorithms exist, and we can, for instance, use the sieve of Eratosthenes to separate prime numbers from non-primes. If we have to verify many numbers, we will not want to run the sieve of Eratosthenes algorithm for every single one of them. What we would like to do instead is tabulate all prime numbers once, up to a certain limit, and use a table lookup to verify a large set of numbers.

In this example, we will generate the C++ code for the lookup table (a vector of prime numbers) by using Python at compile time. Of course, to solve this particular programming problem, we could also generate the lookup table using C++, and we could do it at runtime instead.

Let us start out with the following Python script, called generate.py. This script takes two command-line arguments - an integer that will limit the search, and an output filename:

"""
Generates C++ vector of prime numbers up to max_number
using sieve of Eratosthenes.
"""
import pathlib
import sys

# for simplicity we do not verify argument list
max_number = int(sys.argv[-2])
output_file_name = pathlib.Path(sys.argv[-1])

numbers = range(2, max_number + 1)
is_prime = {number: True for number in numbers}

for number in numbers:
    current_position = number
    if is_prime[current_position]:
        while current_position <= max_number:
            current_position += number
            is_prime[current_position] = False

primes = (number for number in numbers if is_prime[number])
code = """#pragma once

#include <vector>

const std::size_t max_number = {max_number};

std::vector<int> & primes() {{
  static std::vector<int> primes;

{push_back}

  return primes;
}}
"""
push_back = '\n'.join(['  primes.push_back({:d});'.format(x) for x in primes])
output_file_name.write_text(
    code.format(max_number=max_number, push_back=push_back))

Our goal is to generate a header file, primes.hpp, at compile time, and include it in the following example code:

#include "primes.hpp"
  
#include <iostream>
#include <vector>

int main() {
  std::cout << "all prime numbers up to " << max_number << ":";

  for (auto prime : primes())
    std::cout << " " << prime;

  std::cout << std::endl;

  return 0;
}