How Do Local Variables Behave in Panic Recovery in Go?

Understanding Local Variable Assignments in Panic Recovery with Go

Panic recovery in Go allows you to handle runtime errors gracefully, but a common misconception arises when using local variables within the recovery function.

Named Return Value Panic Recovery

When dealing with named return values, the recovery function has access to these variables before they are returned. This enables you to assign values to them within the defer function:

<code class="go">func foo() (result int, err error) {
    defer func() {
        if e := recover(); e != nil {
            result = -1
            err = errors.New(e.(string))
        }
    }()
    bar()
    result = 100
    err = nil
    return
}</code>

Local Variable Assignment in Panic Recovery

However, when using local variables with unnamed return values, this behavior differs. Local variables are created on the stack and initialized with zero values when the function is entered. If the panic occurs before they are assigned, they will retain their zero values.

<code class="go">func foo() (int, error) {
    var result int
    var err error
    defer func() {
        if e := recover(); e != nil {
            result = -1
            err = errors.New(e.(string))
        }
    }()
    bar()
    result = 100
    err = nil
    return result, err
}</code>

In this example, result and err are initialized to 0 and nil, respectively. When the panic occurs before any assignments are made, these zero values are returned. As a result, the output will be:

result: 0

Key Differences

Named return values are treated as named variables, allowing the defer function to modify them directly. Local variables, on the other hand, are stored on the stack and are not accessible to the defer function until they are assigned.

Conclusion

When recovering from panic with local variables, it's crucial to understand that local variables are not initialized until they are assigned. Therefore, if a panic occurs before the assignments, they will retain their zero values and affect the returned values.

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