Go Interfaces

One of the interesting aspects of the Go language is interface objects. In Go, the word interface is overloaded to mean several different things.

Every type has an interface, which is the set of methods defined for that type. This bit of code defines a struct type S with one field, and defines two methods for S.

type S struct { i int }
func (p *S) Get() int { return p.i }
func (p *S) Put(v int) { p.i = v }

You can also define an interface type, which is simply a set of methods. This defines an interface I with two methods:

type I interface {
  Get() int;
  Put(int);
}

S is a valid implementation for I, because it defines the two methods which I requires. Note that this is true even though there is no explicit declaration that S implements I. A Go program can use this fact via yet another meaning of interface, which is an interface value:

func f(p I) { fmt.Println(p.Get()); p.Put(1) }

Here the variable p holds a value of interface type. Because S implements I, we can call f passing in a pointer to a value of type S:

var s S; f(&s)

The reason we need to take the address of S, rather than a value of type S, is because we defined the methods on S to operate on pointers. This is not a requirement—we could have defined the methods to take values—but then the Put method would not work as expected.

The fact that you do not need to declare whether a type implements an interface means that Go implements a form of duck typing. This is not pure duck typing, because when possible the Go compiler will statically check whether the type implements the interface. However, Go does have a purely dynamic aspect, in that you can convert from one interface type to another. In the general case, that conversion is checked at runtime. If the conversion is invalid—if the type of the value stored in the existing interface value does not satisfy the interface to which it is being converted—the program will fail with a runtime error.

For example, since every type satisfies the empty interface interface {}:

func g(i interface{}) int { return i.(I).Get() }
func h() {
  var s S;
  fmt.Println(g(&s));
  fmt.Println(g(s)); // will fail at runtime
}

The first call to g will work fine and will print 0. The second call will fail at runtime; when using gccgo, the program will print

panic: interface conversion failed: no 'Get' method

This is because, as discussed above, a value of type S rather than *S does not have any methods.

So, how does this work? I will describe the current gccgo implementation. The implementation used in the 6g/8g compiler is generally similar but is different in important respects.

For every type which is converted to an interface type, gccgo will build a type descriptor. Type descriptors are used by the type reflection support in the reflect package, and they are also used by the internal runtime support. Type descriptors are defined in the file libgo/runtime/go-type.h in the source code. The relevant part here is that for each type they define a (possibly empty) list of methods. For each method five pieces of information are stored: a hash code for the type of the method (i.e., the parameter and result types); the name of the method; for a method which is not exported, the name of the package in which it is defined; the type descriptor for the type of the method; a pointer to the function which implements the method.

In gccgo, an interface value is really a struct with three fields (libgo/runtime/interface.h): a pointer to the type descriptor for the type of the value currently stored in the interface value; a pointer to a table of functions implementing the methods for the current value; a pointer to the current value itself.

The table of functions is stored sorted by the name of the method, although the name does not appear in the table. Calling a method on an interface means calling a specific entry in this table, where the entry to call is known at compile time. Thus the table of functions is essentially the same as a C++ virtual function table, and calling a method on an interface requires the same series of steps as calling a C++ virtual function: load the address of the virtual table, load a specific entry in the table, and call it.

When a value is statically converted to an interface type, the gccgo compiler will build the table of methods required for that value and that interface type. This table is specific to the pair of types involved. gccgo will give this table comdat linkage, so that it is only built once for each pair of types in a program. Thus a static conversion to an interface simply requires loading the three fields of the interface struct with values known at compile time.

A dynamic conversion from one interface type to another is more complex. Of the three fields in an interface value, the type descriptor and the pointer to the real value can simply be copied to the new interface value. However, the table of methods must be built at runtime. This is done by looking at the list of methods defined in the value’s type descriptors and the list of methods defined in the type descriptor for the interface type itself. Both lists are sorted by the name of the method. The runtime code (libgo/runtime/go-convert-interface.c) merges the two sorted lists to produce the method table. The merging is done using the name of the method and the type hash code. If the interface requires a method which the type does not provide, the conversion fails.

Interfaces in Go are similar to ideas in several other programming languages: pure abstract virtual base classes in C++; typeclasses in Haskell; duck typing in Python; etc. That said, I’m not aware of any other language which combines interface values, static type checking, dynamic runtime conversion, and no requirement for explicitly declaring that a type satisfies an interface. The result in Go is powerful, flexible, efficient, and easy to write.