How to Get a Type Descriptor in Go Without an Instance?

TypeOf without an Instance

When working with Go, it's useful to obtain a type's representation without an existing instance. This can be achieved using reflection techniques.

Utilizing TypeOf with a Typed Nil

The crux lies in starting with a pointer to the desired type and casting it to nil. This creates a typed nil value, allowing us to access the base type using the Elem() method.

<code class="go">t := reflect.TypeOf((*int)(nil)).Elem()
fmt.Println(t) // Output: int</code>

Example Usage

To illustrate, consider the following ParamReader struct:

<code class="go">type ParamReader struct {
    // The request from which to extract parameters
    context *http.Request
}

func (p *ParamReader) Require(s string, t reflect.Type) {
    // Validation logic omitted for brevity
}</code>

Within the Require() method, we can utilize reflect.TypeOf() with typed nil pointers to check parameter types:

<code class="go">if( t == reflect.TypeOf((*uint64)(nil)) {
    // Validation logic for uint64
}</code>

Alternative with Custom Type

Instead of passing raw types, it's convenient to use a custom type that encapsulates specific types. This can be achieved with a simple enum-like structure:

<code class="go">type TypeDesc int

const (
    TypeInt TypeDesc = iota
    TypeHttpRequest
    TypeOsFile
    TypeInt64
)</code>

Utilizing Constants

For efficiency, it's recommended to use predefined constants rather than creating types dynamically.

<code class="go">const (
    TypeInt int = iota
    TypeHttpRequest
    TypeOsFile
    TypeInt64
)</code>

Conclusion

By leveraging reflection techniques and strategic use of typed nil values, it's possible to obtain Type descriptors without an instance. This opens up various possibilities for runtime type checking and validation, as demonstrated in the ParamReader example.

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