Backend DevelopmentGolangHow does a `*extensionsv1beta1.Deployment` variable satisfy the `runtime.Object` type constraint in Kubernetes?
How &deployment Satisfies Type runtime.Object in Kubernetes Code
In the kubectl/run.go file of the Kubernetes code, the Generate function generates a list of values of type runtime.Object. The final line of the function assigns a value of type *extensionsv1beta1.Deployment to the first result variable, which is type annotated as runtime.Object. How does this assignment satisfy the type constraint?
Embbedded Meta and Promoted Methods
The deployment variable is a local variable of type extensionsv1beta1.Deployment. extensionsv1beta1.Deployment embeds metav1.TypeMeta, which includes a GetObjectKind() method. Because extensionsv1beta1.Deployment embeds this type, both extensionsv1beta1.Deployment and *extensionsv1beta1.Deployment (pointers to extensionsv1beta1.Deployment) have promoted versions of this method. Similarly, extensionsv1beta1.Deployment also has a DeepCopyObject() method.
Method Sets and Interface Implementation
runtime.Object is an interface type that specifies a method set, including GetObjectKind() and DeepCopyObject(). An interface type can store a value of any type with a method set that is a superset of the interface's method set. That type is said to implement the interface.
Pointer Receiver Methods
The promoted versions of GetObjectKind() and DeepCopyObject() have pointer receivers. This means that both *extensionsv1beta1.Deployment and &deployment (the pointer to the deployment variable) have the required methods in their method sets.
Type Assignability
Because &deployment implements all the required methods of runtime.Object, it can be assigned to a variable of type runtime.Object. This is consistent with the Go type system rule that an interface type variable can store a value of any type that implements the interface.
Therefore, &deployment satisfies the type requirement of runtime.Object because:
- extensionsv1beta1.Deployment embeds metav1.TypeMeta and has promoted GetObjectKind() and DeepCopyObject() methods.
- Pointers to extensionsv1beta1.Deployment also have these promoted methods.
- &deployment is a pointer to extensionsv1beta1.Deployment, which implements runtime.Object.
- The Go type system permits the assignment of values that implement an interface to variables of that interface type.
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