嵌入何时使用指针

Question

When I want to embed a struct within another struct, should I use a pointer or value?

For example

type Job struct {
    Command string
    *log.Logger
}

or

type Job struct {
    Command string
    log.Logger
}

Answer 1

You can use one or the other: for struct type, the spec mentions:

A field declared with a type but no explicit field name is an anonymous field, also called an embedded field or an embedding of the type in the struct.

An embedded type must be specified as a type name T or as a pointer to a non-interface type name *T, and T itself may not be a pointer type.

Since log.Logger is not an interface, you can use the type or a pointer to the type for the anonymous field Logger.

---

The article "Embedding in Go " fro Eric Urban (hydrogen18) calls embedding a pointer "embed by-pointer":

• The first advantage to this is that you can rely on functions that use the NewX idiom returning a struct by-pointer to do initialization.
• The second advantage is that you can embed all the functionality of a type without needing to know when it is instantiated.
  The embedded pointer to a Bitmap is no different than any other pointer in Go, so it can be assigned multiple times.
  By doing this you can change what instance you are extending dynamically at run time.

For instance, with:

type Bitmap struct{
    data [4][5]bool
}

type Renderer struct{
    *Bitmap //Embed by pointer
    on uint8
    off uint8
}

The Renderer type embeds a Bitmap by-pointer.

A single instance of Bitmap can act as the embedded instance of many Renderer instances:

var renderA,renderB Renderer
renderA.on = 'X'
renderA.off = 'O'
renderB.on = '@'
renderB.off = '.'

var pic Bitmap
pic.data[0][6] = true
pic.data[0][7] = true
pic.data[1][8] = true
pic.data[2][9] = true
pic.data[3][10] = true

renderA.Bitmap = &pic
renderB.Bitmap = &pic

renderA.render()
renderB.render()

This shares the same Bitmap instance to two different renderers.
Each renderer has its own set of characters, allowing two representations of the bitmap to be printed.
This is what the output looks like:

OXXO
OXOO
OXOO
OXOO
.@@.
.@..
.@..
.@..

This example demonstrates the Flyweight Pattern.
Although inconsequential to memory consumption in this example, having many thousands of instances sharing a single underlying data structure can be very significant in reducing the memory consumption of systems.

---

As mentioned in this thread:

The reason why you can't have pointer to pointer and pointer to interface anonymous fields is that these types don't have methods.
The whole point of anonymous fields is that methods get promoted.

I already explained why interfaces don't have methods: a lot of people were using pointers to interfaces incorrectly and unnecessarily, and there weren't any known valid uses, so the language was changed to actively discourage this usage by making pointers to interfaces have no methods.