How Do Blank Identifiers Ensure Interface Implementation at Compile Time in Go?

Blank Identifiers in Variable Assignments: A Compile-Time Assurance

In the context of variables, a blank identifier is used to indicate that the variable will not be used or referenced later in the program. However, it still serves a purpose in the compilation process, as seen in the code snippet:

var _ PropertyLoadSaver = (*Doubler)(nil)

This declaration uses a blank identifier for the variable, making it anonymous and inaccessible. Its true purpose lies in verifying that the *Doubler type adheres to the PropertyLoadSaver interface.

Implementing Interfaces

In Go, a type implements an interface when its method set includes or exceeds the method set of the interface. This means that any type that implements an interface must provide all the methods defined in that interface.

Asserting Interface Implementation

The code above asserts that the Doubler type implements the PropertyLoadSaver interface. If Doubler does not meet this requirement, compilation will halt with an error message indicating that *Doubler does not implement the required method(s) of PropertyLoadSaver.

How it Works

The expression (*Doubler)(nil) converts the untyped nil value to a nil value of type Doubler. This nil value can be assigned to the PropertyLoadSaver variable, but only if Doubler satisfies the interface.

Why Use a Blank Identifier?

A blank identifier is employed because the variable itself is not intended to be utilized. It serves solely to validate the interface implementation during compilation.

Alternative with Non-Blank Identifier

If desired, a non-blank identifier can be used instead, as demonstrated below:

var assertStarDoublerIsPropertyLoadSaver PropertyLoadSaver = (*Doubler)(nil)

While this approach is functionally equivalent, it does introduce a named variable that may not be used elsewhere in the program.

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