Why Does `reflect.Value.FieldByName` Panic When Used on a Pointer Value?

Reflect.Value.FieldByName Panic Explained

When invoking the .FieldByName method on a reflected value, you may encounter a panic error similar to:

<code class="go">panic: reflect: call of reflect.Value.FieldByName on ptr Value</code>

This error typically occurs when you're using reflect.Value incorrectly.

In the example code provided:

<code class="go">s := reflect.ValueOf(&value).Elem()
metric := s.FieldByName(subval.Metric).Interface()</code>

The root of the issue is that value is already a pointer to a struct. By taking the address of &value, you're creating a pointer to pointer. When you then call Elem(), you're dereferencing the pointer to pointer, which isn't necessary.

To resolve this issue, simply use reflect.ValueOf(value).Elem() instead of reflect.ValueOf(&value).Elem(). This will correctly dereference the original pointer, providing you with the actual struct value.

For clarity, below is a modified version of your code:

<code class="go">s := reflect.ValueOf(value).Elem()
metric := s.FieldByName(subval.Metric).Interface()
fmt.Println(metric)</code>

By using reflect.ValueOf(value).Elem(), you obtain the actual struct value, allowing you to access its fields using s.FieldByName.

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