英文:
[]byte(string) vs []byte(*string)
问题
我很好奇为什么Go语言没有提供[]byte(*string)方法。从性能的角度来看，[]byte(string)不是会复制输入参数并增加额外的开销吗（尽管这似乎很奇怪，因为字符串是不可变的，为什么要复制它们）？
英文:
I'm curious as why Go doesn't provide a []byte(*string) method. From a performance perspective, wouldn't []byte(string) make a copy of the input argument and add more cost (though this seems odd since strings are immutable, why copy them)?
答案1
得分: 26
[]byte("something") 不是一个函数（或方法）调用，而是一个类型转换。
类型转换本身不会复制值。但是，将 string 转换为 []byte 会进行复制，这是必要的，因为结果的字节切片是可变的。如果不进行复制，你可以修改/更改 string 值（string 的内容），而 string 是不可变的，正如 规范：字符串类型 部分所规定的那样：
字符串是不可变的：一旦创建，就无法更改字符串的内容。
请注意，有一些情况下，string <=> []byte 的转换不会进行复制，因为编译器会对其进行优化。这些情况很少见，并且是在有证据证明不可变的 string 不会被修改的情况下进行的。
一个例子是从一个键类型为 string 的映射中查找值，并使用转换为 string 的 []byte 进行索引（来源）：
key := []byte("some key")
var m map[string]T
// ...
v, ok := m[string(key)] // 这里复制 key 被优化掉了
另一个优化是在显式转换为字节切片的字符串的字节上进行迭代：
s := "something"
for i, v := range []byte(s) { // 这里复制 s 被优化掉了
// ...
}
（请注意，如果没有进行转换，for range 将迭代字符串的 rune 而不是其 UTF8 编码的字节。）
英文:
[]byte("something") is not a function (or method) call, it's a type conversion.
The type conversion "itself" does not copy the value. Converting a string to a []byte however does, and it needs to, because the result byte slice is mutable, and if a copy would not be made, you could modify / alter the string value (the content of the string) which is immutable, it must be as the Spec: String types section dictates:
> Strings are immutable: once created, it is impossible to change the contents of a string.
Note that there are few cases when string <=> []byte conversion does not make a copy as it is optimized "away" by the compiler. These are rare and "hard coded" cases when there is proof an immutable string cannot / will not end up modified.
Such an example is looking up a value from a map where the key type is string, and you index the map with a []byte, converted to string of course (source):
key := []byte("some key")
var m map[string]T
// ...
v, ok := m[string(key)] // Copying key here is optimized away
Another optimization is when ranging over the bytes of a string that is explicitly converted to a byte slice:
s := "something"
for i, v := range []byte(s) { // Copying s is optimized away
// ...
}
(Note that without the conversion the for range would iterate over the runes of the string and not over its UTF8-encoded bytes.)
答案2
得分: 5
因为这没有意义。
指针（任何类型的指针）不能以任何明显有意义的方式表示为[]byte。
从性能的角度来看，[]byte(string)会复制输入参数并增加更多的开销（尽管这似乎很奇怪，因为字符串是不可变的，为什么要复制它们）。
将[]byte转换为string（反之亦然）确实涉及复制，因为字符串是不可变的，但字节数组不是。
然而，使用指针并不能解决这个问题。
英文:
> I'm curious as why Golang doesn't provide a []byte(*string) method.
Because it doesn't make sense.
A pointer (to any type) cannot be represented (in any obviously meaningful way) as a []byte.
> From a performance perspective, wouldn't []byte(string) make a copy of the input argument and add more cost (though this seems odd since strings are immutable, why copy them)?
Converting from []byte to string (and vice versa) does involve a copy, because strings are immutable, but byte arrays are not.
However, using a pointer wouldn't solve that problem.
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