What's the purpose of using braces (i.e. {}) for a single-line if or loop?

Question

I\'m reading some lecture notes of my C++ lecturer and he wrote the following:

1. Use Indentation // OK
2. Never rely on operator preced

Answer 1

It's very easy to accidentally change control-flow with comments if you do not use { and }. For example:

if (condition)
  do_something();
else
  do_something_else();

must_always_do_this();

If you comment out do_something_else() with a single line comment, you'll end up with this:

if (condition)
  do_something();
else
  //do_something_else();

must_always_do_this();

It compiles, but must_always_do_this() isn't always called.

We had this issue in our code base, where someone had gone in to disable some functionality very quickly before release. Fortunately we caught it in code review.