lawyer

Introduction

Reflection in computing is the ability of a program to examine its own structure, particularly through types; it s a form of metaprogramming. It s also a great source of confusion.

In this article we attempt to clarify things by explaining how reflection works in Go. Each language s reflection model is different (and many languages don t support it at all), but this article is about Go, so for the rest of this article the word reflection should be taken to mean reflection in Go .

Types and interfaces

Because reflection builds on the type system, let s start with a refresher about types in Go.

Go is statically typed. Every variable has a static type, that is, exactly one type known and fixed at compile time: int. float32. *MyType. []byte. and so on. If we declare

then i has type int and j has type MyInt. The variables i and j have distinct static types and, although they have the same underlying type, they cannot be assigned to one another without a conversion.

One important category of type is interface types, which represent fixed sets of methods. An interface variable can store any concrete (non-interface) value as long as that value implements the interface s methods. A well-known pair of examples is io.Reader and io.Writer. the types Reader and Writer from the io package :

Any type that implements a Read (or Write ) method with this signature is said to implement io.Reader (or io.Writer ). For the purposes of this discussion, that means that a variable of type io.Reader can hold any value whose type has a Read method:

It s important to be clear that whatever concrete value r may hold, `r` s type is always io.Reader. Go is statically typed and the static type of r is io.Reader .

An extremely important example of an interface type is the empty interface:

It represents the empty set of methods and is satisfied by any value at all, since any value has zero or more methods.

Some people say that Go s interfaces are dynamically typed, but that is misleading. They are statically typed: a variable of interface type always has the same static type, and even though at run time the value stored in the interface variable may change type, that value will always satisfy the interface.

We need to be precise about all this because reflection and interfaces are closely related.

The representation of an interface

Russ Cox has written a detailed blog post about the representation of interface values in Go. It s not necessary to repeat the full story here, but a simplified summary is in order.

A variable of interface type stores a pair: the concrete value assigned to the variable, and that value s type descriptor. To be more precise, the value is the underlying concrete data item that implements the interface and the type describes the full type of that item. For instance, after

r contains, schematically, the (value, type) pair, ( tty. *os.File ). Notice that the type *os.File implements methods other than Read ; even though the interface value provides access only to the Read method, the value inside carries all the type information about that value. That s why we can do things like this:

The expression in this assignment is a type assertion; what it asserts is that the item inside r also implements io.Writer. and so we can assign it to w. After the assignment, w will contain the pair ( tty. *os.File ). That s the same pair as was held in r. The static type of the interface determines what methods may be invoked with an interface variable, even though the concrete value inside may have a larger set of methods.

Continuing, we can do this:

and our empty interface value empty will again contain that same pair, ( tty. *os.File ). That s handy: an empty interface can hold any value and contains all the information we could ever need about that value.

(We don t need a type assertion here because it s known statically that w satisfies the empty interface. In the example where we moved a value from a Reader to a Writer. we needed to be explicit and use a type assertion because `Writer` s methods are not a subset of `Reader` s.)

One important detail is that the pair inside an interface always has the form (value, concrete type) and cannot have the form (value, interface type). Interfaces do not hold interface values.

Now we re ready to reflect.

The first law of reflection

1. Reflection goes from interface value to reflection object.

At the basic level, reflection is just a mechanism to examine the type and value pair stored inside an interface variable. To get started, there are two types we need to know about in package reflect. Type and Value. Those two types give access to the contents of an interface variable, and two simple functions, called reflect.TypeOf and reflect.ValueOf. retrieve reflect.Type and reflect.Value pieces out of an interface value. (Also, from the reflect.Value it s easy to get to the reflect.Type. but let s keep the Value and Type concepts separate for now.)

Let s start with TypeOf :

This program prints

You might be wondering where the interface is here, since the program looks like it s passing the float64 variable x. not an interface value, to reflect.TypeOf. But it s there; as godoc reports. the signature of reflect.TypeOf includes an empty interface:

When we call reflect.TypeOf(x). x is first stored in an empty interface, which is then passed as the argument; reflect.TypeOf unpacks that empty interface to recover the type information.

The reflect.ValueOf function, of course, recovers the value (from here on we ll elide the boilerplate and focus just on the executable code):

prints

Both reflect.Type and reflect.Value have lots of methods to let us examine and manipulate them. One important example is that Value has a Type method that returns the Type of a reflect.Value. Another is that both Type and Value have a Kind method that returns a constant indicating what sort of item is stored: Uint. Float64. Slice. and so on. Also methods on Value with names like Int and Float let us grab values (as int64 and float64 ) stored inside:

prints

There are also methods like SetInt and SetFloat but to use them we need to understand settability, the subject of the third law of reflection, discussed below.

The reflection library has a couple of properties worth singling out. First, to keep the API simple, the getter and setter methods of Value operate on the largest type that can hold the value: int64 for all the signed integers, for instance. That is, the Int method of Value returns an int64 and the SetInt value takes an int64 ; it may be necessary to convert to the actual type involved:

The second property is that the Kind of a reflection object describes the underlying type, not the static type. If a reflection object contains a value of a user-defined integer type, as in

the Kind of v is still reflect.Int. even though the static type of x is MyInt. not int. In other words, the Kind cannot discriminate an int from a MyInt even though the Type can.

Given a reflect.Value we can recover an interface value using the Interface method; in effect the method packs the type and value information back into an interface representation and returns the result:

As a consequence we can say

to print the float64 value represented by the reflection object v .

(Why not fmt.Println(v). Because v is a reflect.Value ; we want the concrete value it holds.) Since our value is a float64. we can even use a floating-point format if we want:

and get in this case

Again, there s no need to type-assert the result of v.Interface() to float64 ; the empty interface value has the concrete value s type information inside and Printf will recover it.

In short, the Interface method is the inverse of the ValueOf function, except that its result is always of static type interface<> .

The third law is the most subtle and confusing, but it s easy enough to understand if we start from first principles.

Here is some code that does not work, but is worth studying.

prints

It is an error to call a Set method on an non-settable Value. But what is settability?

Settability is a bit like addressability, but stricter. It s the property that a reflection object can modify the actual storage that was used to create the reflection object. Settability is determined by whether the reflection object holds the original item. When we say

we pass a copy of x to reflect.ValueOf. so the interface value created as the argument to reflect.ValueOf is a copy of x. not x itself. Thus, if the statement

were allowed to succeed, it would not update x. even though v looks like it was created from x. Instead, it would update the copy of x stored inside the reflection value and x itself would be unaffected. That would be confusing and useless, so it is illegal, and settability is the property used to avoid this issue.

If this seems bizarre, it s not. It s actually a familiar situation in unusual garb. Think of passing x to a function:

We would not expect f to be able to modify x because we passed a copy of `x` s value, not x itself. If we want f to modify x directly we must pass our function the address of x (that is, a pointer to x ):

This is straightforward and familiar, and reflection works the same way. If we want to modify x by reflection, we must give the reflection library a pointer to the value we want to modify.

Let s do that. First we initialize x as usual and then create a reflection value that points to it, called p .

The output so far is

The reflection object p isn t settable, but it s not p we want to set, it s (in effect) *p. To get to what p points to, we call the Elem method of Value. which indirects through the pointer, and save the result in a reflection Value called v :