Why Can't I Use a `for...range` Loop on a Pointer to a Slice in Go?

Understanding "Golang cannot range over pointer to slice"

When attempting to iterate over a pointer to a slice, especially a slice that holds custom data structures, an error like "cannot range over classes (type *[]entities.Class)" may occur. This error highlights a common misconception when dealing with pointers and slices in Go.

Slices in Go inherently have pointer-like behavior. They reference the underlying array in memory, and any changes made to the slice's elements are reflected in the original array. Therefore, passing a slice to a function effectively provides direct access to the array elements.

However, using a pointer to a slice does not offer any added functionality. Instead, it introduces unnecessary complexity and the potential for confusion.

In the provided code, the ClassesForLastNDays method initializes a new slice, *[]entities.Class, and assigns the value contained in classes to it. This operation essentially copies the slice, and further manipulations on classes do not affect the original list.

To iterate over the contents of the slice correctly, one should use its built-in for-range loop as demonstrated below:

for i := range classes {
    class := classes[i]
}

By avoiding pointers to slices and embracing the natural properties of Go slices, developers can avoid unnecessary errors and maintain code clarity.

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