Certain techniques which make code more maintainable

While contributing to libkiwix. I learned about a few techniques from the maintainers which made code more maintainable.

When are variables justifed?

Only when they increase the readability.Not all variables increase readability.
Example 1

return "http://" + addresses.addr + suffix;

and

 std::string result = "http://";
 result = result + addresses + suffix
 return result

Here the result variable adds no readability. Infact it decreases it.

Example 2

  std::string serverUrl;
  if (port == HTTP_DEFAULT_PORT) {
    serverUrl = host + root;
  } else {
    serverUrl = host + ":" + std::to_string(port) + root;
  }
  return serverUrl;

and

if (port == httpDefaultPort) {
    return "http://" + host + root;
  } else {
    return "http://" + host + ":" + std::to_string(port) + root;
  }

In both the examples, the second code looks more readable.
Though this mostly applies to simple functions.

Declaring const variables wherever possible.

I advise you to start practising defensive programming. A powerful technique is to declare const whatever can be declared const, which protects you against carelessly modifying things that are supposed to be immutable.

C way of defining constants vs the C++ way

#defines are the old C-style way of declaring constants. Better use a const int