Page History

Scope minimization helps developers to avoid common programming errors, improves code readability by tying together connecting the declaration and actual use of a variable, and improves maintainability because unused variables are more easily detected and removed. It may also allow objects to be recovered by the garbage collector more quickly, and it prevents violations of DCL51-J. Do not shadow or obscure identifiers in subscopes.

Noncompliant Code Example

This noncompliant code example shows a variable that is declared outside the for loop. This reduces reusability because the value of the loop index i will have changed after the for statement. Consider for instance, the case when this code snippet is copied and pasted with the intent to use a different index j. If index variable change is omitted, the new loop would then attempt to iterate over index i. Unexpected behavior may follow because i remains in scope. 

public class Scope {
  public static void main(String[] args) {
    int i = 0;
    for (i = 0; i < 10; i++) {
      // Do operations
    }
  }
}

This code is noncompliant because, even though variable i is not intentionally used outside the for loop, it is declared in method scope. One of the few scenarios where variable i would need to be declared in method scope is when the loop contains a break statement and the value of i must be inspected after conclusion of the loop.

Compliant Solution

Minimize the scope of variables where possible, for example by declaring loop indexes . For example, declare loop indices within the for statement. :

public class Scope {
  public static void main(String[] args) {
    for (int i = 0; i < 10; i++) { //contains Contains declaration
      // Do operations
    }
  }
}

Additionally, methods should be designed to perform only one operation if possible. This reduces the need for variables existing in overlapping scopes and consequently, helps prevent errors.

Risk Assessment

Using a larger scope than what is necessary results in less reliable code.

...

Noncompliant Code Example

This noncompliant code example shows a variable count that is declared outside the counter method, although the variable is not used outside the counter method.

public class Foo {
  private int count;
  private static final int MAX_COUNT = 10;

  public void counter() {
    count = 0;
    while (condition()) {
      /* ... */
      if (count++ > MAX_COUNT) {
	    return;
      } 
    }
  }

  private boolean condition() {/* ... *}
  // No other method references count 
  // but several other methods reference MAX_COUNT 
}

Compliant Solution

In this compliant solution, the count field is declared local to the counter() method:

public class Foo {
  private static final int MAX_COUNT = 10;

  public void counter() {
    int count = 0;
    while (condition()) {
      /* ... */
      if (count++ > MAX_COUNT) { 
	    return;
      }
    }
  }
  private boolean condition() {/* ... */}
  // No other method references count 
  // but several other methods reference MAX_COUNT 
}

Applicability

Detecting local variables that are declared in a larger scope than is required by the code as - written code is straightforward , and can avoid any eliminate the possibility of false positives.

Detecting multiple for statements that use the same index variable is straightforward; it will produce produces false positives in the unusual case where this was intended by the programmer.

Other Languages

This guideline appears in the C Secure Coding Standard as DCL19-C. Use as minimal a scope as possible for all variables and functions.

This guideline appears in the C++ Secure Coding Standard as DCL07-CPP. Use as minimal scope as possible for all variables and methods.

Related Vulnerabilities

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

Bibliography

\[[Bloch 2001|AA. Bibliography#Bloch 01]\] Item 29, Minimize the scope of local variables
\[[JLS 2005|AA. Bibliography#JLS 05]\] [Section 14.4.2, Scope of Local Variable Declarations|http://java.sun.com/docs/books/jls/third_edition/html/statements.html#14.4.2]

the value of the index variable is intended to persist between loops.

Bibliography

...

Image Added Image Added Image Added05. Scope (SCP)      05. Scope (SCP)      SCP01-J. Do not increase the accessibility of overridden or hidden methods