Why Don\'t My Struct Updates Persist When Using a Slice in Go?

FTR Struct Update Issue

In Go, when working with structs and slices, it's crucial to understand the concept of referencing and copying.

Consider the provided code, where the user attempts to update values within the ftr struct using a loop. Debugging reveals that while the switch cases are executed and changes are made to the element within the loop, these changes don't persist outside the loop.

The issue stems from the fact that while iterating over the Mod slice, the element obtained is a copy of the actual element in the slice. Thus, any modifications made to this copy won't affect the original value.

Solution

To address this, it's recommended to modify the code as shown below:

for index := range ftr.Mod {
    switch ftr.Mod[index].Type {
    case "aaa", "bbbb":
        ftr.Mod[index].Type = "cccc"
    case "htr":
        ftr.Mod[index].Type = "com"
    case "no":
        ftr.Mod[index].Type = "jnodejs"
    case "jdb":
        ftr.Mod[index].Type = "tomcat"
    }
}

In this modified version, instead of iterating over pointers to elements, we iterate over indices in the slice and directly modify the values of the elements in the slice. This ensures that changes made within the loop are reflected in the original ftr struct.

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