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

There are a number of possible ways of writing control statements; certain combinations of them may co-exist without impairing legibility, but other combinations will cause trouble. The style

if (condition)
  statement;

will co-exist comfortably with some of the other ways of writing control statements, but not so well with others. If multi-line controlled statements are written as:

if (condition)
{
  statement;
  statement;
}

then it will be visually obvious which if statements control a single line and which ones control multiple lines. If, however, multi-line if statements are written as:

if (condition) {
  statement;
  statement;
}

then the likelihood of someone trying to extend a single-statement if constructs without adding the necessary braces may be much higher.

The single-statement-on-next line if statement may also be problematic if the codebase makes significant use of the form

if (condition) statement;

My own preference is that having the statement on its own line generally enhances legibility except in cases where there are many if statements with similar control blocks, e.g.

if (x1 > xmax) x1 = xmax;
if (x1 < xmin) x1 = xmin;
if (x2 > xmax) x2 = xmax;
if (x2 < xmin) x2 = xmin;
etc.

in which case I will generally precede and follow such groups of if statements with a blank line to visually separate them from other code. Having a range of statements that all start with if at the same indentation will then provide a clear visual indication that there's something unusual.