Why Return a Pointer Instead of a Value in Go's 'New' Function?

Understanding the Usage of Address Return in Go Constructors: New vs. Direct Return

Unlike other languages that provide explicit constructors, Go employs a "New" function in place of constructors. However, why is it common to return the address (&f) in Go's "New" func, rather than directly returning the File object?

Differences Between Address and Direct Returns

Go's syntax allows for returning either values (non-pointers) or pointers, leaving the decision to the developer. Typically, a pointer is returned when the value is more useful as one. This is especially the case if the returned value:

1. Has Multiple Methods with Pointer Receivers:

Returning a pointer allows for chaining method calls directly on the returned object, even if the methods take a pointer receiver. This eliminates the need to store the returned value in a variable and manually call the methods.

2. Is Stored in Non-Addressable Data Structures:

Some data structures, such as maps, do not support direct addressing of values. Returning a pointer circumvents this limitation, allowing for method calls on the value stored in the data structure.

3. Is a Large Struct Passed Around Frequently:

Pointers are advantageous for large structs that will be heavily passed around, as it optimizes memory usage and performance.

Example Comparison:

Consider a simple struct with a method that takes a pointer receiver:

type My int

func (m *My) Str() string { return strconv.Itoa(int(*m)) }

func createMy(i int) My { return My(i) }  // Direct return
func createMyPtr(i int) *My { return (*My)(&i) }  // Address return

When attempting to use the direct return value:

fmt.Println(createMy(12).Str())

An error occurs: "cannot call pointer method on createMy(12)". However, with the address return:

fmt.Println(createMyPtr(12).Str())

The method call succeeds without error.

Conclusion:

The decision of whether to return a pointer or value directly in Go's "New" func is based on the intended usage of the returned object. Pointers provide advantages when dealing with methods with pointer receivers, non-addressable data structures, or large structs that will be passed around frequently.

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