Can Nested Classes Exist Within Functions in Go?

Nested Classes within Function in Go: Possibilities and Limitations

In Go, nested structs within functions are permissible, but the ability to create nested functions (beyond lambda functions) is restricted. This raises the question: Is it truly not possible to define a nested class within a function?

To illustrate, consider the following code snippet:

<code class="go">func f() {
    // Nested struct Cls within f
    type Cls struct {
        //...
    }
    // Attempt to bind foo to Cls (fails)
    func (c *Cls) foo() {
        //...
    }
}</code>

In this example, the attempt to define a function foo as a member of the nested struct Cls fails. This limitation may seem unusual, given the flexibility offered by nested structs.

However, it's important to recognize that nesting functions in Go is inherently disallowed, regardless of whether the function has a receiver.

To circumvent this restriction, Go provides function literals, which allow for the declaration of functions within other functions, as seen below:

<code class="go">func f() {
    foo := func(s string) {
        fmt.Println(s)
    }

    foo("Hello World!")
}</code>

In this example, within the outer function f, we create a function variable foo. This variable stores a function that can be called with the argument "Hello World!". When the outer function f is invoked, "Hello World!" is printed as anticipated.

This approach, utilizing function literals, enables the emulation of nested classes within functions, albeit within certain constraints.

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