Anonymous Struct in Go Maps: What\'s the Difference Between `struct{}{} `and `{}`?

Anonymous Struct: Delving into the Differences between struct{}{} and {}

Problem:

In Go, using an anonymous struct as a value in a map declaration raises a warning with Gogland, prompting the question: what's the difference between struct{}{} and {} declarations in such contexts?

Answer:

Understanding the semantics behind these two syntaxes is crucial.

Breaking Down the Declarations

• struct{}{}: This is a composite literal consisting of the anonymous struct type struct{} and the value {}.
• {}: This is also a composite literal, representing just the value {} without explicitly specifying its type.

Type Inference and Composite Literals

Typically, composite literals require an explicit type declaration (e.g., []int{1, 2, 3}). However, when working with maps, the compiler can infer the types of keys and values from the map type itself. This allows the composite literal syntax to be simplified, omitting the type declaration when values of the appropriate type are provided.

Go Specification and Oversight

According to the Go Specification, in cases like these, the type declaration can be elided when the composite literal's type matches the type expected by the map. This behavior was initially an oversight but became a valid feature in Go 1.5.

Note: The type declaration cannot be omitted when initializing other types of composite literals (e.g., arrays, slices).

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