Stringer method requires value

Question

The Go FAQ answers a question regarding the choice of by-value vs. by-pointer receiver definition in methods. One of the statements in that answer is:

If some of the methods of the type must have pointer receivers, the rest should too, so the method set is consistent regardless of how the type is used.

This implies that if I have for a data type a few methods that mutate the data, thus require by-pointer receiver, I should use by-pointer receiver for all the methods defined for that data type.

On the other hand, the "fmt" package invokes the String() method as defined in the Stringer interface by value. If one defines the String() method with a receiver by-pointer it would not be invoked when the associated data type is given as a parameter to fmt.Println (or other fmt formatting methods). This leaves one no choice but to implement the String() method with a receiver by value.

How can one be consistent with the choice of by-value vs. by-pointer, as the FAQ suggests, while fulfilling fmt requirements for the Stringer interface?

EDIT:

In order to emphasize the essence of the problem I mention, consider a case where one has a data type with a set of methods defined with receiver by-value (including String()). Then one wishes to add an additional method that mutates that data type - so he defines it with receiver by-pointer, and (in order to be consistent, per FAQ answer) he also updates all the other methods of that data type to use by-pointer receiver. This change has zero impact on any code that uses the methods of this data type - BUT for invocations of fmt formatting functions (that now require passing a pointer to a variable instead of its value, as before the change). So consistency requirements are only problematic in the context of fmt. The need to adjust the manner one provides a variable to fmt.Println (or similar function) based on the receiver type breaks the capability to easily refactor one's package.

Answer 1

If you define your methods with pointer receiver, then you should use and pass pointer values and not non-pointer values. Doing so the passed value does indeed implement Stringer, and the fmt package will have no problem "detecting" and calling your String() method.

Example:

type Person struct {
    Name string
}

func (p *Person) String() string {
    return fmt.Sprintf("Person[%s]", p.Name)
}

func main() {
    p := &Person{Name: "Bob"}
    fmt.Println(p)
}

Output (try it on the Go Playground):

Person[Bob]

If you would pass a value of type Person to fmt.Println() instead of a pointer of type *Person, yes, indeed the Person.String() would not be called. But if all methods of Person has pointer receiver, that's a strong indication that you should use the type and its values as pointers (unless you don't intend its methods to be used).

Yes, you have to know whether you have to use Person or *Person. Deal with it. If you want to write correct and efficient programs, you have to know a lot more than just whether to use pointer or non-pointer values, I don't know why this is a big deal for you. Look it up if you don't know, and if you're lazy, use a pointer as the method set of (the type of) a pointer value contains methods with both pointer and non-pointer receiver.

Also the author of Person may provide you a NewPerson() factory function which you can rely on to return the value of the correct type (e.g. Person if methods have value receivers, and *Person if the methods have pointer receivers), and so you won't have to know which to use.

Answer to later adding a method with pointer receiver to a type which previously only had methods with value receiver:

Yes, as you described in the question, that might not break existing code, yet continuing to use a non-pointer value may not profit from the later added method with pointer receiver.

We might ask: is this a problem? When the type was used, the new method you just added didn't exist. So the original code made no assumption about its existence. So it shouldn't be a problem.

Second consideration: the type only had methods with value receiver, so one could easily assume that by their use, the value was immutable as methods with value receiver cannot alter the value. Code that used the type may have built on this, assuming it was not changed by its methods, so using it from multiple goroutines may have omitted certain synchronization rightfully.

So I do think that adding a new method with pointer receiver to a type that previously only had methods with value receiver should not be "opaque", the person who adds this new method has the responsibility to either modify uses of this type to "switch" to pointers and make sure the code remains safe and correct, or deal with the fact that non-pointer values will not have this new method.

Tips:

If there's a chance that a type may have mutator methods in the future, you should start creating it with methods with pointer receivers. Doing so you avoid later having to go through the process described above.

Another tip could be to hide the type entirely, and only publish interfaces. Doing so, the users of this type don't have to know whether the interface wraps a pointer or not, it just doesn't matter. They receive an interface value, and they call methods of the interface. It's the responsibility of the package author to take care of proper method receivers, and return the appropriate type that implements the interface. The clients don't see this and they don't depend on this. All they see and use is the interface.

Answer 2

In order to emphasize the essence of the problem I mention, consider a case where one has a data type with a set of methods defined with receiver by-value (including String()). Then one wishes to add an additional method that mutates that data type - so he defines it with receiver by-pointer, and (in order to be consistent, per FAQ answer) he also updates all the other methods of that data type to use by-pointer receiver. This change has zero impact on any code that uses the methods of this data type - BUT for invocations of fmt formatting functions (that now require passing a pointer to a variable instead of its value, as before the change).

This is not true. All interface of it and some of type assertions will be affected as well - that is why fmt is affected. e.g. :

package main

import (
    "fmt"
)

type I interface {
    String() string
}

func (t t) String() string { return "" }

func (p *p) String() string { return "" }

type t struct{}
type p struct{}

func S(i I) {}

func main() {
    fmt.Println("Hello, playground")
    T := t{}
    P := p{}
    _ = P
    S(T)
    //S(P) //fail
}

To understand this from the root, you should know that a pointer method and a value method is different from the very base. However, for convenience, like the omit of ;, golang compiler looks for cases using pointer methods without a pointer and change it back.

As explained here: https://tour.golang.org/methods/6

So back to the orignal question: consistency of pointer methods. If you read the faq more carefully, you will find it is the very last part of considering to use a value or pointer methods. And you can find counter-example in standard lib examples, in container/heap :

// A PriorityQueue implements heap.Interface and holds Items.
type PriorityQueue []*Item

func (pq PriorityQueue) Len() int { return len(pq) }

func (pq PriorityQueue) Less(i, j int) bool {
    // We want Pop to give us the highest, not lowest, priority so we use greater than here.
    return pq[i].priority > pq[j].priority
}

func (pq PriorityQueue) Swap(i, j int) {
    pq[i], pq[j] = pq[j], pq[i]
    pq[i].index = i
    pq[j].index = j
}

func (pq *PriorityQueue) Push(x interface{}) {
    n := len(*pq)
    item := x.(*Item)
    item.index = n
    *pq = append(*pq, item)
}

func (pq *PriorityQueue) Pop() interface{} {
    old := *pq
    n := len(old)
    item := old[n-1]
    item.index = -1 // for safety
    *pq = old[0 : n-1]
    return item
}

// update modifies the priority and value of an Item in the queue.
func (pq *PriorityQueue) update(item *Item, value string, priority int) {
    item.value = value
    item.priority = priority
    heap.Fix(pq, item.index)
}

So, indeed, as the FAQ say, to determine whether to use a pointer methods, take the following consideration in order:

1. Does the method need to modify the receiver? If yes, use a pointer. If not, there should be a good reason or it makes confusion.
2. Efficiency. If the receiver is large, a big struct for instance, it will be much cheaper to use a pointer receiver. However, efficiency is not easy to discuss. If you think it is an issue, profile and/or benchmark it before doint so.
3. Consistency. If some of the methods of the type must have pointer receivers, the rest should too, so the method set is consistent regardless of how the type is used. This, to me, means that if the type shall be used as a pointer (e.g., frequent modify), it should use the method set to mark so. It does not mean one type can only have solely pointer methods or the other way around.

Answer 3

The previous answers here do not address the underlying issue, although the answer from leaf bebop is solid advice.

Given a value, you can in fact invoke either pointer or value receiver methods on it, the compiler will do that for you. However, that does not apply when invoking via interface implementations.

This boils down to this dicussion about interface implementations.

In that discussion the discussion is about implementing interfaces with nil pointers. But the underlying discussion revolves around the same issue: when implementing an interface you must choose the pointer or the value type, and there will be no attempt by the compiler, nor can there be any attempt in golang code, to figure out exactly what type it is, and adjust the interface call accordingly.

So for example, when calling

fmt.Println(object)

you are implementing the arg of type interface{} with object of type X. The fmt code within has no interest in knowing whether the type of object is a pointer type or not. It will not even be able to tell without using reflection. It will simply call String() on whatever type that is.

So if you supplied a value of type X, and there just so happens to be a (*X) String() string method, that does not matter, that method will not be called, it will only type-assert whether that type X implements Stringer, it has no interest if type *X asserts Stringer. Since there is no (X) String() string method, it will move on. It will not attempt to check what X may happen to be, whether it's a pointer type, and if not, whether the associated pointer type implements Stringer, and call that String() method instead.

So this is not really a pointer vs value methods issue, this is an interface implementation issue when implementing interface{} in calls to fmt methods.