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Difference in a function implementation when instantiating an object with different function signatures


Pointers vs. values in parameters and return valuesGolang Pointer and Struct member functionStack vs heap allocation of structs in Go, and how they relate to garbage collectionWhy does the compiler require such a strict match for function signatures?When is the init() function run?Why does putting a pointer in an interface in Go cause reflect to lose the name of the type?Implicit interface conversion in golangReturn different specialised implementations of interfaces from the same function in Go langfunction type parameter match in golangGolang change type of pointer interfaceStrange behaviour when Unmarshalling into struct in GoCall function of specific type in Go






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0















Came across the following differences a function implementations. What is the reasoning behind Example 1 returning a pointer and Example 2 returning an actual object?



type MyInterface interface 
Func (param int) float64 //just random signature


//MyInterfaceImpl implements MyInterface
type MyInterfaceImpl struct


//actual implementation
func (myObj *MyInterfaceImpl) Func(param int) float64
return float64(param)



Example 1: the pointer to the MyInterfaceImpl is returned when function returns an interface



func NewMyInterface() MyInterface 
return &MyInterfaceImpl



Example 2: actual object of MyInterfaceImpl is returned when function returns the object



func NewMyInterfaceImpl() MyInterfaceImpl 
return MyInterfaceImpl



UPDATE: This piece of code compiles and runs



func main() 
myIf := NewMyInterface()
fmt.Printf("Hi from inteface %fn", myIf.Func(1000))

myImpl := NewMyInterfaceImpl()
fmt.Printf("Hi from impl %fn", myImpl.Func(100))



UPDATE2: Question clarification.



This sounds weird (for me) to have a declaration of func NewMyInterface() MyInterface and a valid implementation of return &MyInterfaceImpl where a pointer is returned. I would expect to return an object of MyInterfaceImpl with return MyInterfaceImpl



If the language allows such types of constructs, there must be a reason for that. Eventually, I am looking for a following answer: "The function declaration returns an interface. Because the interface has a property X it is does not make sense to return an object, but the only valid option is a pointer".










share|improve this question
























  • Are you sure its not NewMyInterface() *MyInterface { for the first example?

    – Ibu
    Mar 8 at 22:02











  • @lbu Updated the description

    – Timofey
    Mar 8 at 22:13












  • Possible duplicate of Golang Pointer and Struct member function

    – munk
    Mar 8 at 22:21






  • 1





    What is the reasoning behind Example 1 returning a pointer and Example 2 returning an actual object? depends what they do, their members, how they are being used. With current examples its hard to tell. Now, as a rule of thumb, take interface, return structs. to read medium.com/@cep21/…

    – mh-cbon
    Mar 8 at 22:49






  • 1





    @mh-cbon Example 1 has to return a pointer, as MyInterfaceImpl doesn't implement MyInterface (despite the name...) But I agree it's hard to tell what the question is.

    – ᆼᆺᆼ
    Mar 8 at 22:55


















0















Came across the following differences a function implementations. What is the reasoning behind Example 1 returning a pointer and Example 2 returning an actual object?



type MyInterface interface 
Func (param int) float64 //just random signature


//MyInterfaceImpl implements MyInterface
type MyInterfaceImpl struct


//actual implementation
func (myObj *MyInterfaceImpl) Func(param int) float64
return float64(param)



Example 1: the pointer to the MyInterfaceImpl is returned when function returns an interface



func NewMyInterface() MyInterface 
return &MyInterfaceImpl



Example 2: actual object of MyInterfaceImpl is returned when function returns the object



func NewMyInterfaceImpl() MyInterfaceImpl 
return MyInterfaceImpl



UPDATE: This piece of code compiles and runs



func main() 
myIf := NewMyInterface()
fmt.Printf("Hi from inteface %fn", myIf.Func(1000))

myImpl := NewMyInterfaceImpl()
fmt.Printf("Hi from impl %fn", myImpl.Func(100))



UPDATE2: Question clarification.



This sounds weird (for me) to have a declaration of func NewMyInterface() MyInterface and a valid implementation of return &MyInterfaceImpl where a pointer is returned. I would expect to return an object of MyInterfaceImpl with return MyInterfaceImpl



If the language allows such types of constructs, there must be a reason for that. Eventually, I am looking for a following answer: "The function declaration returns an interface. Because the interface has a property X it is does not make sense to return an object, but the only valid option is a pointer".










share|improve this question
























  • Are you sure its not NewMyInterface() *MyInterface { for the first example?

    – Ibu
    Mar 8 at 22:02











  • @lbu Updated the description

    – Timofey
    Mar 8 at 22:13












  • Possible duplicate of Golang Pointer and Struct member function

    – munk
    Mar 8 at 22:21






  • 1





    What is the reasoning behind Example 1 returning a pointer and Example 2 returning an actual object? depends what they do, their members, how they are being used. With current examples its hard to tell. Now, as a rule of thumb, take interface, return structs. to read medium.com/@cep21/…

    – mh-cbon
    Mar 8 at 22:49






  • 1





    @mh-cbon Example 1 has to return a pointer, as MyInterfaceImpl doesn't implement MyInterface (despite the name...) But I agree it's hard to tell what the question is.

    – ᆼᆺᆼ
    Mar 8 at 22:55














0












0








0


0






Came across the following differences a function implementations. What is the reasoning behind Example 1 returning a pointer and Example 2 returning an actual object?



type MyInterface interface 
Func (param int) float64 //just random signature


//MyInterfaceImpl implements MyInterface
type MyInterfaceImpl struct


//actual implementation
func (myObj *MyInterfaceImpl) Func(param int) float64
return float64(param)



Example 1: the pointer to the MyInterfaceImpl is returned when function returns an interface



func NewMyInterface() MyInterface 
return &MyInterfaceImpl



Example 2: actual object of MyInterfaceImpl is returned when function returns the object



func NewMyInterfaceImpl() MyInterfaceImpl 
return MyInterfaceImpl



UPDATE: This piece of code compiles and runs



func main() 
myIf := NewMyInterface()
fmt.Printf("Hi from inteface %fn", myIf.Func(1000))

myImpl := NewMyInterfaceImpl()
fmt.Printf("Hi from impl %fn", myImpl.Func(100))



UPDATE2: Question clarification.



This sounds weird (for me) to have a declaration of func NewMyInterface() MyInterface and a valid implementation of return &MyInterfaceImpl where a pointer is returned. I would expect to return an object of MyInterfaceImpl with return MyInterfaceImpl



If the language allows such types of constructs, there must be a reason for that. Eventually, I am looking for a following answer: "The function declaration returns an interface. Because the interface has a property X it is does not make sense to return an object, but the only valid option is a pointer".










share|improve this question
















Came across the following differences a function implementations. What is the reasoning behind Example 1 returning a pointer and Example 2 returning an actual object?



type MyInterface interface 
Func (param int) float64 //just random signature


//MyInterfaceImpl implements MyInterface
type MyInterfaceImpl struct


//actual implementation
func (myObj *MyInterfaceImpl) Func(param int) float64
return float64(param)



Example 1: the pointer to the MyInterfaceImpl is returned when function returns an interface



func NewMyInterface() MyInterface 
return &MyInterfaceImpl



Example 2: actual object of MyInterfaceImpl is returned when function returns the object



func NewMyInterfaceImpl() MyInterfaceImpl 
return MyInterfaceImpl



UPDATE: This piece of code compiles and runs



func main() 
myIf := NewMyInterface()
fmt.Printf("Hi from inteface %fn", myIf.Func(1000))

myImpl := NewMyInterfaceImpl()
fmt.Printf("Hi from impl %fn", myImpl.Func(100))



UPDATE2: Question clarification.



This sounds weird (for me) to have a declaration of func NewMyInterface() MyInterface and a valid implementation of return &MyInterfaceImpl where a pointer is returned. I would expect to return an object of MyInterfaceImpl with return MyInterfaceImpl



If the language allows such types of constructs, there must be a reason for that. Eventually, I am looking for a following answer: "The function declaration returns an interface. Because the interface has a property X it is does not make sense to return an object, but the only valid option is a pointer".







go






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Mar 8 at 23:10







Timofey

















asked Mar 8 at 21:38









TimofeyTimofey

1,61923143




1,61923143












  • Are you sure its not NewMyInterface() *MyInterface { for the first example?

    – Ibu
    Mar 8 at 22:02











  • @lbu Updated the description

    – Timofey
    Mar 8 at 22:13












  • Possible duplicate of Golang Pointer and Struct member function

    – munk
    Mar 8 at 22:21






  • 1





    What is the reasoning behind Example 1 returning a pointer and Example 2 returning an actual object? depends what they do, their members, how they are being used. With current examples its hard to tell. Now, as a rule of thumb, take interface, return structs. to read medium.com/@cep21/…

    – mh-cbon
    Mar 8 at 22:49






  • 1





    @mh-cbon Example 1 has to return a pointer, as MyInterfaceImpl doesn't implement MyInterface (despite the name...) But I agree it's hard to tell what the question is.

    – ᆼᆺᆼ
    Mar 8 at 22:55


















  • Are you sure its not NewMyInterface() *MyInterface { for the first example?

    – Ibu
    Mar 8 at 22:02











  • @lbu Updated the description

    – Timofey
    Mar 8 at 22:13












  • Possible duplicate of Golang Pointer and Struct member function

    – munk
    Mar 8 at 22:21






  • 1





    What is the reasoning behind Example 1 returning a pointer and Example 2 returning an actual object? depends what they do, their members, how they are being used. With current examples its hard to tell. Now, as a rule of thumb, take interface, return structs. to read medium.com/@cep21/…

    – mh-cbon
    Mar 8 at 22:49






  • 1





    @mh-cbon Example 1 has to return a pointer, as MyInterfaceImpl doesn't implement MyInterface (despite the name...) But I agree it's hard to tell what the question is.

    – ᆼᆺᆼ
    Mar 8 at 22:55

















Are you sure its not NewMyInterface() *MyInterface { for the first example?

– Ibu
Mar 8 at 22:02





Are you sure its not NewMyInterface() *MyInterface { for the first example?

– Ibu
Mar 8 at 22:02













@lbu Updated the description

– Timofey
Mar 8 at 22:13






@lbu Updated the description

– Timofey
Mar 8 at 22:13














Possible duplicate of Golang Pointer and Struct member function

– munk
Mar 8 at 22:21





Possible duplicate of Golang Pointer and Struct member function

– munk
Mar 8 at 22:21




1




1





What is the reasoning behind Example 1 returning a pointer and Example 2 returning an actual object? depends what they do, their members, how they are being used. With current examples its hard to tell. Now, as a rule of thumb, take interface, return structs. to read medium.com/@cep21/…

– mh-cbon
Mar 8 at 22:49





What is the reasoning behind Example 1 returning a pointer and Example 2 returning an actual object? depends what they do, their members, how they are being used. With current examples its hard to tell. Now, as a rule of thumb, take interface, return structs. to read medium.com/@cep21/…

– mh-cbon
Mar 8 at 22:49




1




1





@mh-cbon Example 1 has to return a pointer, as MyInterfaceImpl doesn't implement MyInterface (despite the name...) But I agree it's hard to tell what the question is.

– ᆼᆺᆼ
Mar 8 at 22:55






@mh-cbon Example 1 has to return a pointer, as MyInterfaceImpl doesn't implement MyInterface (despite the name...) But I agree it's hard to tell what the question is.

– ᆼᆺᆼ
Mar 8 at 22:55













1 Answer
1






active

oldest

votes


















1














Even though I'm not sure which part of the code the question is about, let me explain what the code does:



MyInterface is implemented by anything having a Func(int)float64 method.
*MyInterfaceImpl has such a method. However, MyInterfaceImpl does not (the method has a pointer receiver).



NewMyInterface() thus has to return a pointer. MyInterfaceImpl wouldn't implement MyInterface.



Does this answer your question?




Another question might be why the call myImpl.Func(100) works, despite the above. This is because Go automatically takes the address of the receiver when calling its methods with pointer receivers.

This is explained in more detail for example here.






share|improve this answer

























  • Could you please clarify why MyInterfaceImpl does not implement MyInterface? It sounds like I am missing a fundamental go knowledge here.

    – Timofey
    Mar 8 at 23:17






  • 1





    The spec explains it. The method set for a value type does not include pointer receiver methods.

    – Cerise Limón
    Mar 8 at 23:18











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1 Answer
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active

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1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









1














Even though I'm not sure which part of the code the question is about, let me explain what the code does:



MyInterface is implemented by anything having a Func(int)float64 method.
*MyInterfaceImpl has such a method. However, MyInterfaceImpl does not (the method has a pointer receiver).



NewMyInterface() thus has to return a pointer. MyInterfaceImpl wouldn't implement MyInterface.



Does this answer your question?




Another question might be why the call myImpl.Func(100) works, despite the above. This is because Go automatically takes the address of the receiver when calling its methods with pointer receivers.

This is explained in more detail for example here.






share|improve this answer

























  • Could you please clarify why MyInterfaceImpl does not implement MyInterface? It sounds like I am missing a fundamental go knowledge here.

    – Timofey
    Mar 8 at 23:17






  • 1





    The spec explains it. The method set for a value type does not include pointer receiver methods.

    – Cerise Limón
    Mar 8 at 23:18















1














Even though I'm not sure which part of the code the question is about, let me explain what the code does:



MyInterface is implemented by anything having a Func(int)float64 method.
*MyInterfaceImpl has such a method. However, MyInterfaceImpl does not (the method has a pointer receiver).



NewMyInterface() thus has to return a pointer. MyInterfaceImpl wouldn't implement MyInterface.



Does this answer your question?




Another question might be why the call myImpl.Func(100) works, despite the above. This is because Go automatically takes the address of the receiver when calling its methods with pointer receivers.

This is explained in more detail for example here.






share|improve this answer

























  • Could you please clarify why MyInterfaceImpl does not implement MyInterface? It sounds like I am missing a fundamental go knowledge here.

    – Timofey
    Mar 8 at 23:17






  • 1





    The spec explains it. The method set for a value type does not include pointer receiver methods.

    – Cerise Limón
    Mar 8 at 23:18













1












1








1







Even though I'm not sure which part of the code the question is about, let me explain what the code does:



MyInterface is implemented by anything having a Func(int)float64 method.
*MyInterfaceImpl has such a method. However, MyInterfaceImpl does not (the method has a pointer receiver).



NewMyInterface() thus has to return a pointer. MyInterfaceImpl wouldn't implement MyInterface.



Does this answer your question?




Another question might be why the call myImpl.Func(100) works, despite the above. This is because Go automatically takes the address of the receiver when calling its methods with pointer receivers.

This is explained in more detail for example here.






share|improve this answer















Even though I'm not sure which part of the code the question is about, let me explain what the code does:



MyInterface is implemented by anything having a Func(int)float64 method.
*MyInterfaceImpl has such a method. However, MyInterfaceImpl does not (the method has a pointer receiver).



NewMyInterface() thus has to return a pointer. MyInterfaceImpl wouldn't implement MyInterface.



Does this answer your question?




Another question might be why the call myImpl.Func(100) works, despite the above. This is because Go automatically takes the address of the receiver when calling its methods with pointer receivers.

This is explained in more detail for example here.







share|improve this answer














share|improve this answer



share|improve this answer








edited Mar 8 at 23:31

























answered Mar 8 at 23:09









ᆼᆺᆼᆼᆺᆼ

8,67943563




8,67943563












  • Could you please clarify why MyInterfaceImpl does not implement MyInterface? It sounds like I am missing a fundamental go knowledge here.

    – Timofey
    Mar 8 at 23:17






  • 1





    The spec explains it. The method set for a value type does not include pointer receiver methods.

    – Cerise Limón
    Mar 8 at 23:18

















  • Could you please clarify why MyInterfaceImpl does not implement MyInterface? It sounds like I am missing a fundamental go knowledge here.

    – Timofey
    Mar 8 at 23:17






  • 1





    The spec explains it. The method set for a value type does not include pointer receiver methods.

    – Cerise Limón
    Mar 8 at 23:18
















Could you please clarify why MyInterfaceImpl does not implement MyInterface? It sounds like I am missing a fundamental go knowledge here.

– Timofey
Mar 8 at 23:17





Could you please clarify why MyInterfaceImpl does not implement MyInterface? It sounds like I am missing a fundamental go knowledge here.

– Timofey
Mar 8 at 23:17




1




1





The spec explains it. The method set for a value type does not include pointer receiver methods.

– Cerise Limón
Mar 8 at 23:18





The spec explains it. The method set for a value type does not include pointer receiver methods.

– Cerise Limón
Mar 8 at 23:18



















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