When Should You Use Parentheses for Struct Initialization in Go?

Understanding Parenthesized Struct Initialization in Go

In Go, struct initialization is typically done using curly braces { and }, as seen in the following example:

<code class="go">item1 := Item{1, "Foo"}</code>

However, it is also possible to initialize structs using parentheses (), as shown below:

<code class="go">item2 := (Item{2, "Bar"})</code>

Impact of Parenthesized Initialization:

Both forms of initialization result in identical Item values. Parenthesized initialization does not provide any specific advantages or changes in functionality.

When to Use Parentheses:

Parentheses become necessary when initializing structs within certain control structures, such as if statements. Without parentheses, the compiler may encounter a parsing ambiguity, resulting in an error. For instance:

<code class="go">if i := Item{3, "a"}; i.Id == 3 {
}</code>

This code will cause a compile error because the compiler is unsure whether the opening brace { belongs to the composite literal or the body of the if statement. To resolve this ambiguity, parentheses can be added:

<code class="go">if i := (Item{3, "a"}); i.Id == 3 {
}</code>

The parentheses clearly indicate that the composite literal is a complete expression, which is then assigned to the variable i within the if statement.

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