Why Does Go Return 'cannot range over pointer to slice' and How Can I Fix It?

Error: Cannot Range Over Pointer to Slice in Golang

When attempting to iterate over a pointer to a slice in Golang, an "cannot range over slice pointer" error may occur. This is a common misconception, as pointers to slices are generally unnecessary.

In the provided code snippet, the issue arises in the populateClassRelationships function:

func (c *ClassRepository) populateClassRelationships(classes *[]entities.Class) {
    for i := range classes {  <---------- Here is the problem

The type of the classes parameter is a pointer to a slice (*[]entities.Class), but the range-over syntax expects a direct slice type ([]entities.Class).

Reason

Go slices are already pointers to the underlying array, making pointers to slices technically redundant and inefficient. A slice contains the following information:

• A pointer to the first element of the array
• The length of the slice
• The capacity of the slice (the maximum number of elements it can hold)

As such, assigning a slice to a pointer does not add any value and can create confusion.

Solution

To resolve the error, simply remove the indirection (*). The correct syntax for iterating over a slice is:

func (c *ClassRepository) populateClassRelationships(classes []entities.Class) {
    for i := range classes {

This will directly iterate over the elements of the classes slice without referencing a pointer to the slice.

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