月份：2016年3月

以下摘自The Go Programming Language：

A named struct type S can’t declare a field of the same type S: an aggregate value cannot contain itself. (An analogous restriction applies to arrays.) But S may declare a field of the pointer type *S, which lets us create recursive data structures like linked lists and trees.

The struct type with no fields is called the empty struct, written struct{}. It has size zero and carries no information but may be useful nonetheless. Some Go programmers use it instead of bool as the value type of a map that represents a set, to emphasize that only the keys are significant, but the space saving is marginal and the syntax more cumbersome, so we generally avoid it.

Go lets us declare a field with a type but no name; such fields are called anonymous fields. The type of the field must be a named type or a pointer to a named type.

In Go, a map is a reference to a hash table, and a map type is written map[K]V, where K and V are the types of its keys and values. All of the keys in a given map are of the same type, and all of the values are of the same type, but the keys need not be of the same type as the values. The key type K must be comparable using ==, so that the map can test whether a given key is equal to one already within it. Though floating-point numbers are comparable, it’s a bad idea to compare floats for equality, especially bad if NaN is a possible value. There are no restrictions on the value type V. The built-in function make can be used to create a map.

A map element is not a variable, and we cannot take its address:
_ = &ages[“bob”] // compile error: cannot take address of map element
One reason that we can’t take the address of a map element is that growing a map might cause rehashing of existing elements into new storage locations, thus potentially invalidating the address.

The zero value for a map type is nil, that is, a reference to no hash table at all.
var ages map[string]int
fmt.Println(ages == nil) // “true”
fmt.Println(len(ages) == 0) // “true”
Most operations on maps, including lookup, delete, len, and range loops, are safe to perform on a nil map reference, since it behaves like an empty map. But storing to a nil map causes a panic:
ages[“carol”] = 21 // panic: assignment to entry in nil map
You must allocate the map before you can store into it.

As with slices, maps cannot be compared to each other; the only legal comparison is with nil.

Sometimes we need a map or set whose keys are slices, but because a map’s keys must be comparable, this cannot be expressed directly. However, it can be done in two steps. First we define a helper function k that maps each key to a string, with the property that k(x) == k(y) if and only if we consider x and y equivalent. Then we create a map whose keys are strings, applying the helper function to each key before we access the map.
The same approach can be used for any non-comparable key type, not just slices. It’s even useful for comparable key types when you want a definition of equality other than ==, such as case-insensitive comparisons for strings. And the type of k(x) needn’t be a string; any comparable type with the desired equivalence property will do, such as integers, arrays, or structs.

A slice has three components: a pointer, a length, and a capacity. The pointer points to the first element of the array that is reachable through the slice, which is not necessarily the array’s first element. The length is the number of slice elements; it can’t exceed the capacity, which is usually the number of elements between the start of the slice and the end of the underlying array. The built-in functions len and cap return those values.Multiple slices can share the same underlying array and may refer to overlapping parts of that array：

The slice operator s[i:j], where 0 ≤ i ≤ j ≤ cap(s), creates a new slice that refers to elements i through j-1 of the sequence s, which may be an array variable, a pointer to an array, or another slice. The resulting slice has j-i elements. If i is omitted, it’s 0, and if j is omitted, it’s len(s).

Since a slice contains a pointer to an element of an array, passing a slice to a function permits the function to modify the underlying array elements. In other words, copying a slice creates an alias for the underlying array.

Unlike arrays, slices are not comparable, so we cannot use == to test whether two slices contain the same elements. The standard library provides the highly optimized bytes.Equal function for comparing two slices of bytes ([]byte), but for other types of slice, we must do the comparison ourselves.

The only legal slice comparison is against nil, as in
if summer == nil { /* … */ }
The zero value of a slice type is nil. A nil slice has no underlying array. The nil slice has length and capacity zero, but there are also non-nil slices of length and capacity zero, such as []int{} or make([]int, 3)[3:]. As with any type that can have nil values, the nil value of a particular slice type can be written using a conversion expression such as []int(nil).

var s []int // len(s) == 0, s == nil

s = nil // len(s) == 0, s == nil

s = []int(nil) // len(s) == 0, s == nil

s = []int{} // len(s) == 0, s != nil

So, if you need to test whether a slice is empty, use len(s) == 0, not s == nil. Other than comparing equal to nil, a nil slice behaves like any other zero-length slice.