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

Panic Triggered by Reflect.Value.FieldByName

When attempting to utilize the .FieldByName method on a reflected value, a panic can arise. Specifically, the error message reads:

panic: reflect: call of reflect.Value.FieldByName on ptr Value

Delving into the issue, the relevant code snippet appears as follows:

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

Explanation:

The error occurs due to an unnecessary operation in the code. The value is already a pointer to a struct. By taking its address and then calling Elem(), the code effectively dereferences the pointer. As a result, the resulting reflect.Value is a pointer, on which the .FieldByName method is not applicable.

Solution:

To address this issue, simply remove the redundant step of taking the address of value. The corrected code should appear as:

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

This modification ensures that s is a reference to the struct itself, rather than a pointer, allowing the .FieldByName method to be called successfully.

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