go - 在结构中取消引用指向 DB 的指针

Question

通常，当我看到在结构上声明的字段时，它没有指针或取消引用的指针符号 *，但是在几个代码片段中，我看到结构中的数据库字段带有指针取消引用，如下所示。为什么有必要？

type DB struct {
    *bolt.DB
}
func Open(path string, mode os.FileMode) (*DB, error) {
    db, err := bolt.Open(path, mode)
    if err != nil {
        return nil, err
    }
    return &DB{db}, nil
}

Answer 1

or a dereferenced pointer symbol *

这是规范，对于复杂的非值类型，为了避免复制。
参见 Golang book "Pointers"例如其中带有指针的结构。

return &DB{db}

返回指向新创建的 DB 实例的指针。
如“Can you “pin” an object in memory with Go?”中所述:

Note that, unlike in C, it's perfectly OK to return the address of a local variable; the storage associated with the variable survives after the function returns

来自“Pointer/Value Subtleties”:

Go is also pass by value, but it has both pointers and value types. Pointers refer to a certain memory location, and allow you to mutate the data at that location

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有关更多信息，请参阅“Best practice “returning” structs in Go?”

Use pointers for big structs or structs you'll have to change, and otherwise use values, because getting things changed by surprise via a pointer is confusing.