m := make(map[string]int, 5)
发现 len(m) = 0
之后我觉得有必要重新复习一下 make
函数了。
make 的作用:分配内存,初始化,返回值(不是指针)。
PS:返回值的说明:返回的是这三种引用类型本身(指针),而不是指向这三个引用的指针。
PS:new 就会返回指针。还有,new 不限类型,不初始化。
slice
-> len, capmap
-> 预留大约 n 个元素的空间(具体分配空间的规则不清楚)chan
-> buffer size
The built-in function make
takes a type T, which must be a slice, map or channel type, optionally followed by a type-specific list of expressions. It returns a value of type T (not *T). The memory is initialized as described in the section on initial values.
Call Type T Result
make(T, n) slice slice of type T with length n and capacity n
make(T, n, m) slice slice of type T with length n and capacity m
make(T) map map of type T
make(T, n) map map of type T with initial space for approximately n elements
make(T) channel unbuffered channel of type T
make(T, n) channel buffered channel of type T, buffer size n
Each of the size arguments n and m must be of integer type or an untyped constant. A constant size argument must be non-negative and representable by a value of type int; if it is an untyped constant it is given type int. If both n and m are provided and are constant, then n must be no larger than m. If n is negative or larger than m at run time, a run-time panic occurs.
s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
s := make([]int, 10, 0) // illegal: len(s) > cap(s)
c := make(chan int, 10) // channel with a buffer size of 10
m := make(map[string]int, 100) // map with initial space for approximately 100 elements
Calling make with a map type and size hint n will create a map with initial space to hold n map elements. The precise behavior is implementation-dependent.