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PWR082: Remove unused variables

Issue​

A variable that is declared but never used adds noise: it increases the amount of information to keep track of, suggesting data flow that does not actually exist. In some cases, an unused variable may instead hint at a hidden defect in the program's logic, such as a missing assignment or an incomplete computation.

Actions​

Determine whether each unused variable is accidentally or intentionally unused; remove those that are not needed. If a variable is intentionally unused but, for any reason, it cannot be removed, make that intent explicit using language-specific mechanisms.

Relevance​

Fortran, C, and C++ allow variables to be declared without ever being used. Eliminating unused variables reduces cognitive load and avoids suggesting nonexistent data flow, helping readers focus on what actually contributes to the program's behavior.

If a variable x must remain intentionally unused (e.g., an argument required in an API call but unused in a particular implementation), it is possible to document that intent using language constructs, such as:

  • C: [[maybe_unused]] int x (since C23), or compiler-specific annotations such as gcc's __attribute__((unused)). Another common alternative, although less explicit, is to "consume" the variable with a cast (void) x.

  • C++: [[maybe_unused]] int x (since C++17).

  • Fortran: There is no standard attribute, and compilers such as gfortran do not provide attributes for annotation. A simple and portable approach is to create a reference with no runtime effect:

associate(unused => x)
end associate

Code examples​

C​

In the following code, the variable t is declared but never used to perform the computation:

// example.c
__attribute__((pure)) int sumArray(const int n, const int *array) {
  int t, sum = 0;

  for (int i = 0; i < n; ++i) {
    sum += array[i];
  }

  return sum;
}

Thus, t should be removed:

// solution.c
__attribute__((pure)) int sumArray(const int n, const int *array) {
  int sum = 0;

  for (int i = 0; i < n; ++i) {
    sum += array[i];
  }

  return sum;
}

Fortran​

In the following code, the variable t is declared but never used to perform the computation:

! example.f90
pure function sumArray(array) result(sum)
  implicit none
  integer, intent(in) :: array(:)
  integer :: t, sum, i

  sum = 0

  do i = 1, size(array)
    sum = sum + array(i)
  end do
end function sumArray

Thus, t should be removed:

! solution.f90
pure function sumArray(array) result(sum)
  implicit none
  integer, intent(in) :: array(:)
  integer :: sum, i

  sum = 0

  do i = 1, size(array)
    sum = sum + array(i)
  end do
end function sumArray

References​