EXP19-C. Use braces for if, for, or while statement

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;

The programmer adds a debugging statement to determine when the login is valid, but forgets to add opening and closing braces.

int login;

if (invalid_login())
  login = 0;
else
  printf("Login is valid\n");  /* debugging line added here */
  login = 1;

Due to the indentation of the code, it is difficult to tell that the code is not functioning as intended by the programmer, leading to a possible security breach.

Compliant Solution

Opening and closing braces are used even when the body is a single statement.

int login;

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

Noncompliant Code Example

When you have an if-else statement nested in another if statement, always put braces around the if-else.

This noncompliant code example does not use braces.

if (a)
  if (b)
    win();
  else
    lose();

Compliant Solution

if (a) {
  if (b) {    
    win();  
  } else {    
    lose();  
  }
}

Noncompliant Code Example

Macros can be used to execute a sequence of multiple statements as group.

In the event that multiple statements in a macro is not bound in a do-while loop (see PRE10-C. Wrap multi-statement macros in a do-while loop), an if statement with opening and closing braces will still ensure that all statements of the macro are properly executed.

/*
 * Swaps two values.
 * Requires tmp variable to be defined.
 */
#define SWAP(x, y) \
  tmp = x; \
  x = y; \
  y = tmp

This macro will expand correctly in a normal sequence of statements, but not as the then-clause in an if statement:

int x, y, z, tmp;
if (z == 0)
  SWAP( x, y);

This will expand to:

int x, y, z, tmp;
if (z == 0)
  tmp = x;
x = y;
y = tmp;

Compliant Solution

Given an if statement bounded with opening and closing braces, the macro would expand as intended.

int x, y, z, tmp;
if (z == 0) {
  tmp = x;
  x = y;
  y = tmp;
}