What's the Purpose of Blank Identifiers in Go's Variable Assignment for Interface Compliance?

Understanding the Role of Blank Identifiers in Variable Assignment

In the realm of Go programming, encountering variable declarations like var _ PropertyLoadSaver = (*Doubler)(nil) might raise questions about their purpose. This declaration is not meant to initialize a variable that can be accessed later. Instead, it serves a more subtle purpose related to compile-time assertions.

Purpose of Blank Identifiers in Assignment

The blank identifier _ signifies that the declared variable is not intended for use. Its primary role is to facilitate a compile-time check that ensures that the *Doubler type adheres to the PropertyLoadSaver interface.

Interface Implementation Verification

An interface defines a set of methods that a specific type must implement. If a type does not fulfill the required methods, compilation will result in an error. In this case, the assignment var _ PropertyLoadSaver = (*Doubler)(nil) acts as a compile-time assertion.

If *Doubler does not implement the PropertyLoadSaver interface, compilation will fail with an error message indicating that the "Doubler does not implement PropertyLoadSaver (missing Save method)."

Under the Hood

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

Alternative with Non-Blank Identifier

While the blank identifier is commonly used, one can also declare a non-blank identifier:

var assertStarDoublerIsPropertyLoadSaver PropertyLoadSaver = (*Doubler)(nil)

In this case, the variable assertStarDoublerIsPropertyLoadSaver is declared but not used, fulfilling the same purpose as the blank identifier. However, using a blank identifier helps to maintain a consistent naming convention and emphasizes that the variable is not intended for accessing elsewhere in the package.

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