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 precedence - Always use parentheses // OK
3. Always use a { } block - even for a single line // not OK, why ???
4. Const object on left side of comparison // OK
5. Use unsigned for variables that are >= 0 // nice trick
6. Set Pointer to NULL after deletion - Double delete protection // not bad

The 3rd technique is not clear to me: what would I gain by placing one line in a { ... }?

For example, take this weird code:

int j = 0;
for (int i = 0 ; i < 100 ; ++i)
{
    if (i % 2 == 0)
    {
        j++;
    }
}

and replace it with:

int j = 0;
for (int i = 0 ; i < 100 ; ++i)
    if (i % 2 == 0)
        j++;

What's the benefit of using the 1st version?

Answer 1

Let's attempt to also modify i when we increment j:

int j = 0;
for (int i = 0 ; i < 100 ; ++i)
    if (i % 2 == 0)
        j++;
        i++;

Oh no! Coming from Python, this looks ok, but in fact it isn't, as it's equivalent to:

int j = 0;
for (int i = 0 ; i < 100 ; ++i)
    if (i % 2 == 0)
        j++;
i++;

Of course, this is a silly mistake, but one that even an experienced programmer could make.

Another very good reason is pointed out in ta.speot.is's answer.

A third one I can think of is nested if's:

if (cond1)
   if (cond2) 
      doSomething();

Now, assume you now want to doSomethingElse() when cond1 is not met (new feature). So:

if (cond1)
   if (cond2) 
      doSomething();
else
   doSomethingElse();

which is obviously wrong, since the else associates with the inner if.

---

Edit: Since this is getting some attention, I'll clarify my view. The question I was answering is:

What's the benefit of using the 1st version?

Which I have described. There are some benefits. But, IMO, "always" rules don't always apply. So I don't wholly support

Always use a { } block - even for a single line // not OK, why ???

I'm not saying always use a {} block. If it's a simple enough condition & behavior, don't. If you suspect someone might come in later & change your code to add functionality, do.