以下摘自The Go Programming Language

Either the receiver argument has the same type as the receiver parameter, for example both have type T or both have type *T:

Point{1, 2}.Distance(q) // Point
pptr.ScaleBy(2) // *Point

Or the receiver argument is a variable of type T and the receiver parameter has type *T. The compiler implicitly takes the address of the variable:

p.ScaleBy(2) // implicit (&p)

Or the receiver argument has type *T and the receiver parameter has type T. The compiler implicitly dereferences the receiver, in other words, loads the value:

pptr.Distance(q) // implicit (*pptr)

If all the methods of a named type T have a receiver type of T itself (not *T), it is safe to copy instances of that type; calling any of its methods necessarily makes a copy.

 

The type of an anonymous field may be a pointer to a named type, in which case fields and methods are promoted indirectly from the pointed-to object. Adding another level of indirection lets us share common structures and vary the relationships between objects dynamically.

 

Usually we select and call a method in the same expression, as in p.Distance(), but it’s possible to separate these two operations. The selector p.Distance yields a method value, a function that binds a method (Point.Distance) to a specific receiver value p. This function can then be invoked without a receiver value; it needs only the non-receiver arguments.

Related to the method value is the method expression. When calling a method, as opposed to an ordinary function, we must supply the receiver in a special way using the selector syntax. A method expression, written T.f or (*T).f where T is a type, yields a function value with a regular first parameter taking the place of the receiver, so it can be called in the usual way.

 

A variable or method of an object is said to be encapsulated if it is inaccessible to clients of the object. Encapsulation, sometimes called information hiding, is a key aspect of object-oriented programming.

Go has only one mechanism to control the visibility of names: capitalized identifiers are exported from the package in which they are defined, and uncapitalized names are not. The same mechanism that limits access to members of a package also limits access to the fields of a struct or the methods of a type. As a consequence, to encapsulate an object, we must make it a struct.

Another consequence of this name-based mechanism is that the unit of encapsulation is the package, not the type as in many other languages. The fields of a struct type are visible to all code within the same package. Whether the code appears in a function or a method makes no difference.

Encapsulation provides three benefits. First, because clients cannot directly modify the object’s variables, one need inspect fewer statements to understand the possible values of those variables.

Second, hiding implementation details prevents clients from depending on things that might change, which gives the designer greater freedom to evolve the implementation without breaking API compatibility.

The third benefit of encapsulation, and in many cases the most important, is that it prevents clients from setting an object’s variables arbitrarily. Because the object’s variables can be set only by functions in the same package, the author of that package can ensure that all those functions maintain the object’s internal invariants.