通过值或指针访问另一个结构

Question

What difference there is when you access to another struct by value or by a pointer?
When should be used each one of them?

type foo_ struct {
    st uint8
    nd uint8
}

type bar struct {
    rd  uint8
    foo foo_
}

type barP struct {
    rd  uint8
    foo *foo_
}

Answer 1

If you declare or allocate a variable of type bar, you reserve and initialize to zero memory for both rd uint8 and foo foo_. There is always one variable of type foo_ embedded in a variable of type bar.

var b bar  // declare b

If you declare or allocate a variable of type barP, you reserve and initialize to zero memory for both rd uint8 and foo *foo_. A zero value pointer is a nil pointer. No variable of type foo_ is allocated; you must do that separately. There is either zero (foo == nil) or one variable of type foo_ pointed to by a variable of type barP. A variable of type barP may point to the same variable of type foo_ as other variables of type barP, sharing the same copy of the variable of type foo_. A change to a shared copy is seen by all variables that point to it.

var bp barP         // declare bp
bp.foo = new(foo_)  // allocate bp.foo

Which one to use depends on the properties of type bar versus type barP. Which type more closely reflects the problem that you are trying to solve?

For example, consider this invoice problem. We always have a billing address; we are always going to ask for our money. However, we often ship to the billing address, but not always. If the shipping address is nil, use the billing address. Otherwise, use a separate shipping address. We have two warehouses, and we always ship from one or the other. We can share the two warehouse locations. Since we don't send an invoice until the order is shipped from the warehouse, the warehouse location will never be nil.

type address struct {
    street string
    city   string
}

type warehouse struct {
    address string
}

type invoice struct {
    name      string
    billing   address
    shipping  *address
    warehouse *warehouse
}