54. Use braces for the body of an if, for, or while statement

Use opening and closing braces for if, for, and while statements even when the body contains only a single statement. Braces improve the uniformity and readability of code.

More important, it is easy to forget to add braces when inserting additional statements into a body containing only a single statement, because the conventional indentation gives strong (but misleading) guidance to the structure.

Noncompliant Code Example

This noncompliant code example authenticates a user with an if statement that lacks braces:

int login;

if (invalid_login())
  login = 0;
else
  login = 1;

This program behaves as expected. However, a maintainer might subsequently add a debug statement or other logic but forget to add opening and closing braces:

int login;

if (invalid_login())
  login = 0;
else
  // Debug line added belov
  System.out.println("Login is valid\n");
  // The next line is always executed 
  login = 1; 

The code's indentation disguises the functionality of the program, potentially leading to a security breach.

Compliant Solution

This compliant solution uses opening and closing braces even though the body of the if and else bodies of the if statement are single statements:

int login;

if (invalid_login()) {
  login = 0;
} else {
  login = 1;
}

Noncompliant Code Example

This noncompliant code example nests an if statement within another if statement, without braces around the if and else bodies:

int privileges;

if (invalid_login())
  if (allow_guests())
    privileges = GUEST;
else
  privileges = ADMINISTRATOR;

The indentation might lead the programmer to believe users are granted administrator privileges only when their login is valid. However, the else statement actually binds to the inner if statement:

int privileges;

if (invalid_login())
  if (allow_guests())
    privileges = GUEST;
  else
    privileges = ADMINISTRATOR;

Consequently, this defect allows unauthorized users to obtain administrator privileges.

Compliant Solution

This compliant solution uses braces to remove the ambiguity, consequently ensuring that privileges are correctly assigned:

int privileges;

if (invalid_login()) {
  if (allow_guests()) {
    privileges = GUEST;
  } 
} else {
  privileges = ADMINISTRATOR;
}

Applicability

Failure to enclose the bodies of if, for, or while statements in braces makes code error prone and increases maintenance costs.

Bibliography