Opening and closing braces for if, for, or while statements should always be used, even if said statement has only a single body line.
Braces help improve the uniformity, and therefore readability of code.
More importantly, when inserting an additional statement in a body containing only a single line, it is easy to forget to add braces when the indentation tends to give a strong (but probably misleading) guide to the structure.
Noncompliant Code Example
This noncompliant code example uses an if-else statement without braces to authenticate a user.
int login; if (invalid_login()) login = 0; else login = 1;