One of the most common points of confusion for developers, especially those new to languages like Go, is the distinction between an interface and a pointer to an interface. The error message “
Understanding Interfaces and Pointers
In many statically typed languages, an interface defines a set of methods that a type must implement to be considered of that interface type. It’s essentially a contract. A pointer, on the other hand, is a variable that holds the memory address of another variable. When we talk about a “pointer to an interface,” we’re dealing with the memory address of a variable that holds an interface value. This difference is critical. The language expects an interface value, which carries both the type information and the actual data, not just the address where that value is stored.
The error message “is pointer to interface, not interface” indicates that you are passing the address of an interface variable where the compiler expects the interface value itself. This typically happens when you’re trying to assign or pass a pointer to a variable or function that expects a concrete type that implements the interface, not a pointer to the interface. To resolve this, you generally need to dereference the pointer, effectively accessing the interface value it points to. This allows the compiler to correctly identify the type information and data associated with the interface.
Consider this analogy: Imagine an interface as a document stating requirements to enter a building. A pointer to that interface is like having the address of the building where the document is stored, not the document itself. You need the actual document (the interface value) to enter, not just the location where it’s kept (the pointer to the interface).
Common Scenarios and Examples
Let’s examine common scenarios where you might encounter this issue. One frequent case involves passing an interface pointer to a function expecting an interface value. Another occurs when assigning a pointer to an interface to a variable declared as an interface type. Understanding these scenarios will help you anticipate and prevent the error.
Scenario 1: Function Arguments. Assume you have a function defined as func processData(data MyInterface). If you have a pointer to MyInterface like dataPtr := &myStruct, where myStruct implements MyInterface, you cannot directly call processData(dataPtr). You must dereference the pointer: processData(dataPtr). This passes the actual interface value to the function.
Scenario 2: Variable Assignment. If you have a variable declared as var myInterface MyInterface, and you have a pointer myInterfacePtr := &myStruct, you must assign the dereferenced value: myInterface = myInterfacePtr. Simply assigning myInterface = myInterfacePtr will result in the “is pointer to interface, not interface” error.
Example: Suppose you have an interface Speaker with a method Speak(). You have a struct Dog that implements Speaker. If you try to pass a Dog to a function expecting a Speaker, the compiler will likely complain if it’s expecting the interface directly and not a pointer to it. You may need to dereference it first, or adjust the function signature depending on the logic.
Resolving the “ is pointer to interface, not interface” Error
The primary solution is to ensure that you’re working with the interface value itself, not a pointer to it. Here are the steps you can take to resolve this error:
- Dereference the Pointer: Use the operator to access the value the pointer points to. This retrieves the actual interface value.
- Review Function Signatures: Ensure the function expects the interface type directly, not a pointer to the interface. Adjust the function signature if necessary.
- Check Variable Assignments: Verify that you’re assigning an interface value to a variable declared as an interface type, not a pointer.
Consider the following code snippet (Go example):
go type MyInterface interface { DoSomething() } type MyStruct struct {} func (m MyStruct) DoSomething() {} func processInterface(i MyInterface) { i.DoSomething() } func main() { myStruct := &MyStruct{} //Correct way processInterface(myStruct) //Incorrect way //processInterface(&myStruct) //This would cause an error } The key takeaway is to understand the difference between the interface value and the pointer to that value. The error arises when the compiler expects the former but receives the latter. By dereferencing the pointer when necessary, you provide the compiler with the correct type information, resolving the error and allowing your code to compile and run correctly. According to research presented in “The Go Programming Language Specification” [^1^], understanding type conversions and interface satisfaction is paramount to writing correct Go code.
Best Practices and Common Pitfalls
To avoid this error in the future, adopt these best practices:
- Explicitly Define Interfaces: Clearly define interfaces to ensure that types implementing them are well-understood.
- Use Interfaces Effectively: Leverage interfaces for abstraction and polymorphism.
Conversely, avoid these common pitfalls:
- Overusing Pointers: Don’t use pointers unnecessarily, especially when dealing with interfaces.
- Ignoring Compiler Warnings: Pay close attention to compiler warnings, as they often point to type mismatches.
One common pitfall is assuming that a pointer to a concrete type automatically satisfies an interface. While a concrete type might implement an interface, a pointer to that type might not directly satisfy it, especially if the interface methods are defined on the value receiver and not the pointer receiver. For example, if Dog implements Speaker with func (d Dog) Speak(), then Dog will not automatically satisfy Speaker. This subtle distinction is important to remember. According to a Stack Overflow survey [^2^], type-related errors are among the most common issues developers face.
Here is a featured snippet-optimized paragraph: The “
- Why does this error occur?
- The error arises because the language expects an interface value (containing type information and data), not just the memory address of that value (a pointer).
- How do I fix this error?
- Dereference the pointer using the operator to access the interface value it points to.
- When is it appropriate to use a pointer to an interface?
- Pointers to interfaces are useful when you want to modify the underlying object through the interface, or when dealing with nil interfaces \[[Go Tour: Methods and interfaces](https://go.dev/tour/methods/4)\].
This error, while initially confusing, is a great opportunity to deepen your understanding of how interfaces and pointers work together. Explore further by experimenting with different scenarios, studying code examples, and referring to language documentation. Consider diving deeper into topics like reflection and dynamic dispatch to further refine your grasp of these concepts. Also, don’t hesitate to explore advanced topics like dependency injection and mock testing, where interfaces play a crucial role. If you found this helpful, check out our article on understanding nil interfaces or another post about effective debugging techniques for similar issues.
[^1^]: Go Programming Language Specification. (n.d.). https://go.dev/ref/spec [^2^]: Stack Overflow Developer Survey. (n.d.). https://survey.stackoverflow.co/ Question & Answer :
I have this problem which seems a bit weird to me. Take a look at this snippet of code:
package coreinterfaces type FilterInterface interface { Filter(s *string) bool } type FieldFilter struct { Key string Val string } func (ff *FieldFilter) Filter(s *string) bool { // Some code } type FilterMapInterface interface { AddFilter(f *FilterInterface) uuid.UUID RemoveFilter(i uuid.UUID) GetFilterByID(i uuid.UUID) *FilterInterface } type FilterMap struct { mutex sync.Mutex Filters map[uuid.UUID]FilterInterface } func (fp *FilterMap) AddFilter(f *FilterInterface) uuid.UUID { // Some code } func (fp *FilterMap) RemoveFilter(i uuid.UUID) { // Some code } func (fp *FilterMap) GetFilterByID(i uuid.UUID) *FilterInterface { // Some code }
On some other package, I have the following code:
func DoFilter() { fieldfilter := &coreinterfaces.FieldFilter{Key: "app", Val: "152511"} filtermap := &coreinterfaces.FilterMap{} _ = filtermap.AddFilter(fieldfilter) // <--- Exception is raised here }
The run-time won’t accept the line mentioned because
“cannot use fieldfilter (type *coreinterfaces.FieldFilter) as type *coreinterfaces.FilterInterface in argument to fieldint.AddFilter: *coreinterfaces.FilterInterface is pointer to interface, not interface”
However, when changing the code to:
func DoBid() error { bs := string(b) var ifilterfield coreinterfaces.FilterInterface fieldfilter := &coreinterfaces.FieldFilter{Key: "app", Val: "152511"} ifilterfield = fieldfilter filtermap := &coreinterfaces.FilterMap{} _ = filtermap.AddFilter(&ifilterfield) }
Everything is alright and when debugging the application it really seems to include
I’m a bit confused on this topic. When looking at other blog posts and stack overflow threads discussing this exact same issue (for example - This, or This) the first snippet which raises this exception should work, because both fieldfilter and fieldmap are initialized as pointers to interfaces, rather than value of interfaces. I haven’t been able to wrap my head around what actually happens here that I need to change in order for me not to declare a FieldInterface and assign the implementation for that interface. There must be an elegant way to do this.
So you’re confusing two concepts here. A pointer to a struct and a pointer to an interface are not the same. An interface can store either a struct directly or a pointer to a struct. In the latter case, you still just use the interface directly, not a pointer to the interface. For example:
type Fooer interface { Dummy() } type Foo struct{} func (f Foo) Dummy() {} func main() { var f1 Foo var f2 *Foo = &Foo{} DoFoo(f1) DoFoo(f2) } func DoFoo(f Fooer) { fmt.Printf("[%T] %+v\n", f, f) }
Output:
[main.Foo] {} [*main.Foo] &{}
https://play.golang.org/p/I7H_pv5H3Xl
In both cases, the f variable in DoFoo is just an interface, not a pointer to an interface. However, when storing f2, the interface holds a pointer to a Foo structure.
Pointers to interfaces are almost never useful. In fact, the Go runtime was specifically changed a few versions back to no longer automatically dereference interface pointers (like it does for structure pointers), to discourage their use. In the overwhelming majority of cases, a pointer to an interface reflects a misunderstanding of how interfaces are supposed to work.
However, there is a limitation on interfaces. If you pass a structure directly into an interface, only value methods of that type (ie. func (f Foo) Dummy(), not func (f *Foo) Dummy()) can be used to fulfill the interface. This is because you’re storing a copy of the original structure in the interface, so pointer methods would have unexpected effects (ie. unable to alter the original structure). Thus the default rule of thumb is to store pointers to structures in interfaces, unless there’s a compelling reason not to.
Specifically with your code, if you change the AddFilter function signature to:
func (fp *FilterMap) AddFilter(f FilterInterface) uuid.UUID
And the GetFilterByID signature to:
func (fp *FilterMap) GetFilterByID(i uuid.UUID) FilterInterface
Your code will work as expected. fieldfilter is of type *FieldFilter, which fullfills the FilterInterface interface type, and thus AddFilter will accept it.
Here’s a couple of good references for understanding how methods, types, and interfaces work and integrate with each other in Go:
