Every declaration should be for a single variable, on its own line, with an explanatory comment about the role of the variable. Declaring multiple variables in a single declaration can cause confusion regarding the types of the variables and their initial values. If more than one variable is declared in a declaration, care must be taken to ensure that the type and initialized value of the variable is self evident.

Noncompliant Code Example

In this noncompliant code example, a programmer or code reviewer might mistakenly conceive that the two variables {{src}} and {{c}} are declared as {{int}}. In fact, {{src}} is of type {{int \[\]}}, while {{c}} has a type of {{int}}.

int src[], c;

Another fallout of this example is that it declares the array in a largely antiquated and unpopular style, with the brackets appearing after the variable name as in type name[]. In practice, arrays are typically declared as type[] name.

Compliant Solution

In this compliant solution, each variable is declared on a separate line. It also uses the preferable style for declaring arrays.

int[] src;   /* source array */
int c;       /* max value    */

Although this change has no effect on compilation, the programmer's intent is clearer.

Noncompliant Example

In this noncompliant example, a programmer or code reviewer might mistakenly believe that both i and j have been initialized to 1. In fact, only j has been initialized, while i remains uninitialized.

int i, j = 1;

Compliant Solution

In this compliant solution, it is readily apparent that both i and j have been initialized to 1.

int i = 1;
int j = 1;

Nomcompliant Example

In this noncompliant example, the original programmer declared multiple variables, including an array, on the same line. All instances of the type T have access to methods of the class Object. However, it is easy to miss that arrays need special treatment when some of these methods are overridden. Oversights of this genre typically go undetected by compilers and IDEs, alike.

public class Example{
  private T a,b,c[],d;

  public Example(T in){
    a = in;
    b = in;
    c = (T[]) new Object[10];
    d = in;
  }

As a result, when a method of Object like toString() is overridden, a programmer might accidentally provide a general implementation for type T without realizing that c is an array.

// The oversight error leads to an incorrect implementation
public String toString(){
  return a.toString() + b.toString() + c.toString() + d.toString();
}

However, the real intent might have been to invoke toString() on each individual member of the type T, in array c.

// Correct functional implementation
public String toString(){
  String s = a.toString() + b.toString();
  for(int i = 0; i < c.length; i++){
    s += c[i].toString();
  }
  s += d.toString();
  return s;
}

Compliant Solution

To be compliant, move each declaration to a different line. Furthermore, declare arrays by placing the brackets adjacent to the type, as opposed to using the postfix notation.

public class Example {
  private T a;
  private T b;
  private T[] c;
  private T d;

  public Example(T in){
    a = in;
    b = in;
    c = (T[]) new Object[10];
    d = in;
  }
}

Exceptions

DCL04-01: Trivial declarations for loop counters, for example, can reasonably be included within a for statement:

for (int i = 0; i < mx; ++i ) {
  /* ... */
}

Risk Assessment

Failing to declare no more than one variable per declaration can affect code readability and cause misinterpretations.

Other Languages

This rule appears in the C Secure Coding Standard as DCL04-C. Do not declare more than one variable per declaration.

This rule appears in the C++ Secure Coding Standard as DCL04-CPP. Do not declare more than one variable per declaration.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[JLS 05|AA. Java References#JLS 05]\] Section 6.1, "Declarations", Section 4.3.2, "The class Object"
\[[ESA 05|AA. Java References#ESA 05]\] Rule 9: Put single variable definitions in separate lines.