example.cpp

#if defined(LOCAL)
#include "logging.h"
#define L(x...) (debug(x, #x))
#else
#define L(x, ...) (x)
#endif

#include <bits/stdc++.h>
using namespace std;

int main() {
  int a, b;
  cin >> a >> b;
  if (L(a + b, a, b) == 42) {
    cout << "well done!" << endl;
  }
}

logging.h

#ifndef H_LOGGING
#define H_LOGGING
// https://louisdx.github.io/cxx-prettyprint/
#include "prettyprint.hpp"

template<typename T>
void _out_to_stream(std::ostream& ss, T&& arg) {
  ss << std::forward<T>(arg);
}

template<typename T, typename... O>
void _out_to_stream(std::ostream& ss, T&& arg, O&&... args) {
  ss << std::forward<T>(arg) << ' ';
  _out_to_stream(ss, std::forward<O>(args)...);
}

template<typename T, typename... O>
decltype(auto) debug(T&& arg, O&&... args) {
  std::stringstream ss;
  _out_to_stream(ss, std::forward<T>(arg), std::forward<O>(args)...);
  std::cerr << ss.str() << std::endl;
  return std::forward<T>(arg);
}

#endif  // H_LOGGING