How Interfaces Work in Go

本文深入探讨了Go语言中接口类型的内部实现机制,包括接口类型的定义、非接口值转换为接口类型、接口值之间的转换、类型断言以及方法调用等关键概念。

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This article will introduce the internal implementation of interface types by the official Go compiler.

The explanations in this article aren't precisely in accordance with the official Go compiler and runtime implementation. The main intention of this article is to explain the theory, instead of concrete implementations. This article also ignores many details, including:

  • garbage collection,
  • synchronization,
  • address alignment.

The Internal Definition Of Interface Types

All interface types have the same internal definition:

type _interface struct {
	dynamicTypeInfo *_implementation
	dynamicValue    unsafe.Pointer // unsafe.Pointer means
	                               // *ArbitraryType in Go.
}

The internal _implementation type is declared like

type _implementation struct {
	itype   *_type   // the interface type.
	dtype   *_type   // the dynamic type, which must implement itype.
	methods []*_func // the methods which are defined on dtype
	                 // and each of them implements a
	                 // corresponding method declared in itype.
}

From the definitions, we know that each interface value contains two pointer fields. The dynamicValue field stores the dynamic value information, and the dynamicTypeInfo field stores the implementation information. dynamicTypeInfo.itype stores the type information of the interface value and dynamicTypeInfo.dtype stores the type information of the dynamic value.

The dynamicTypeInfo field of an interface value may be nil, which means nothing is stored in the interface value. For this case, the dynamicValue field must be also nil. We can also say the dynamic value of the interface value is untyped nil for this case.

For the official Go compiler and runtime, a non-nil dynamicValue field value may store

  • the address of the dynamic value if the dynamic type is not a pointer type, or
  • the dynamic value itself if the dynamic type is a pointer type.

Surely, it is not essential to make the exception for pointer dynamic values. This is just a compiler optimization. We can get why it is an optimization in following sections. (BTW, about more current and future optimizations in the official interface implementation, please read this article.)

Other involved internal types are declared like:

type _func struct {
	name      string  
	methodSig uint // two methods with the same signature have
	               // the same signature id. Receiver parameter
	               // doesn't contribute to this signature.
	funcSig   uint // receiver parameter accounts to this signature.

	// other information ...
}

type _type struct {
	name       string   // type name
	id         uint32   // each type has unique id
	flags      uint32   // comparable? isPointer?
	size       uintptr  // value size in bytes
	kind       uint8    // 
	methods    []*_func // the methods are sorted 
	                    // by methodSig and name.
	// more information ...
}

const (
	// flags
	TypeFlag_Comparable = 1 << 0
	TypeFlag_IsPointer  = 1 << 1
	TypeFlag_IsString   = 1 << 2
)

func (t *_type) IsComparable() bool {
	return t.flags & TypeFlag_Comparable != 0
}

func (t *_type) IsPointer() bool {
	return t.flags & TypeFlag_IsPointer != 0
}

func (t *_type) IsString() bool {
	return t.flags & TypeFlag_IsString != 0
}

Some fields of above types are not listed, for they are unrelated to this article.

Here is the function to get an _implementation value from an interface type and a non-interface type:

// global table
var cachedImpls = map[uint64]*_implementation{}

// itype must be an interface type and
// dtype must be a non-interface type.
// Return nil if dtype doesn't implement itype.
// Must not return nil if dtype implements itype.
func getImpl (itype *_type, dtype *_type) *_implementation {
	var key = uint64(itype.id) << 32 | uint64(dtype.id)
	var impl = cachedImpls[key]
	if impl == nil {
		// for each (dtype, itype) pair, the implementation
		// method table is only calculated most once at
		// run time. The calculation result will be cached.
		
		var numMethods = len(itype.methods)
		var methods = make([]*_func, numMethods)
		
		// find every implemented methods.
		// The methods of itype and dtype are both sorted
		// by methodSig and name.
		var n = 0
		var i = 0
		for _, im := range itype.methods {
			for i < len(dtype.methods) {
				tm := dtype.methods[i]
				i++
				
				// Here, for simplicity, assume
				// all methods are exported.
				
				if tm.methodSig < im.methodSig {
					continue
				}
				if tm.methodSig > im.methodSig {
					// im method is not implemented
					return nil
				}
				if tm.name < im.name {
					continue
				}
				if tm.name > im.name {
					// im method is not implemented
					return nil
				}
				
				methods[n] = tm
				n++
				break
			}
		}
		
		// dtype doesn't implement all methods of itype
		if n < numMethods {
			return nil
		}
		
		// dtype implements itype.
		// create and cache the implementation.
		impl = &_implementation{
			dtype: dtype, 
			itype: itype, 
			methods: methods,
		}
		cachedImpls[key] = impl
	}
	
	return impl
}

This function will be called in the value conversions explained in following sections.

In any Go program, at run time, all _implementation values are cached and stored in a global map and all _type values are stored in a global immutable array.

As the blank interface type interface{} is used popular in Go programming, the official Go compiler uses a different and more efficient underlying definition for the blank interface than other interface types:

// blank interface
struct {
	dynamicType     *_type         // the dynamic type
	dynamicValuePtr unsafe.Pointer // points to the dynamic value
}

To make the explainations simpler, following sections will treat blank interface types as general interface types.

Convert Non-Interface Values To Interface Types

Here is the internal function to convert a non-interface value to an interface type:

// To call this function, compilers must assure 
// 1. itype is an interface type.
// 2. dtype is nil or a non-interface type and implements itype.
// p must be nil if dtype is nil.
// p is a pointer stores the address of a value of dtype if dtype
// is not a pointer type. p stores the value of a value of dtype
// if dtype is a pointer type.
func t2i (itype *_type, dtype *_type, p unsafe.Pointer) _interface {
	// dtype is nil means the non-interface value is untyped nil
	if dtype == nil {
		return _interface {
			dynamicValue:    nil,
			dynamicTypeInfo: nil,
		}
	}
	
	// the pointer dynamic value optimization, no need to
	// allocate the extra memory block.
	if dtype.IsPointer() {
		return _interface {
			dynamicValue:    p,
			dynamicTypeInfo: getImpl(dtype, itype),
		}
	}
	
	// for non-pointer dynamic value, runtime must
	// allocate an extra memory block to store a copy
	// of the non-pointer value.
	var t = memoryAlloc(dtype) 
	memoryCopy(t, p, dtype.size)
	return _interface {
		dynamicValue:    t,
		dynamicTypeInfo: getImpl(dtype, itype),
	}
}

Compilers will insert a call of this function before

  • assigning a non-interface value to an interface value, to convert the non-interface value to the type of the interface value.
  • comparing a non-interface value with an interface value, to convert the non-interface value to the type of the interface value.

Convert Interface Values To Other Interface Types

Here is the internal function to convert an interface value to an interface type:

// To call this function, compilers must assure 
// 1. itype is an interface type.
// 2. the dynamic value of ivalue is untyped nil
//    or the dynamic type of ivalue implements ivalue.
//    (the method set of the dynamic type of ivalue must
//    must be a super set of the method set of itype).
func i2i (itype *_type, ivalue _interface) _interface {
	// the dynamic value of ivalue is untyped nil.
	if ivalue.dynamicTypeInfo == nil {
		return _interface {
			dynamicValue:    nil,
			dynamicTypeInfo: nil,
		} // <=> return ivalue
	}
	
	// compilers should avoid calling this function
	// for this case.
	if ivalue.dynamicTypeInfo.itype == itype {
		return ivalue // return a copy of ivalue.
	}
	
	// Convert the dynamic value of ivalue to itype.
	// Here, the returned interface value and ivalue
	// will share the same extra memory block which
	// stores the dyanmic value if the dynamic value
	// is not a pointer.
	return _interface {
		dynamicValue:    ivalue.dynamicValue,
		dynamicTypeInfo: getImpl(
			ivalue.dynamicTypeInfo.dtype,
			itype,
		), // here, the getImpl call never return nil.
	}
}

Compilers will call this function before

  • assigning an interface value to another interface value, to convert the first interface value to the type of the second interface value.
  • comparing an interface value with another interface value, to convert the first interface value to the type of the second interface value.

Compilers should translate converting an interface value to its own type as a no-op.

Assign Interface Values

In an interface value assignment, the destination value must be an interface value, and the type of the source value must implement the destination interface type. The source value may be either a non-interface value or an interface value. As above two sections mentioned, compilers will convert the source value to the destination interface type before the assignment. So in the final assignment, the source value and the destination value have the same type, the destination interface type.

For the current official Go compiler/runtime, there are just two copies for the two fields, dynamicValue and dynamicTypeInfo, in the final assignment. So if the dynamic value is non-pointer, the underlying dynamic value memory block, which address is stored in the dynamicValue field, will be shared between the destination and source interface values. However, this should be viewed as an optimization. Other compilers may not adopt this optimization.

Compare Interface Values

There are three comparison circumstances involving interface values:

  • interface value vs. interface value.
  • interface value vs. non-interface value.
  • interface value vs. untyped nil.

A good compiler should treat the three circumstances differently to get better program performance. Here, for simplicity, we assume non-interface and untyped nil values will be converted to interface{} type before making the comparisons. So all comparisons involving interface values can be viewed as comparing two interface values.

Here is the internal function to compare two interface values:

func iCompare (ivalue1 _interface, ivalue2 _interface) bool {
	// untyped nil is only equal to untyped nil.
	if ivalue1.dynamicTypeInfo == nil {
		return ivalue2.dynamicTypeInfo == nil
	}
	if ivalue2.dynamicTypeInfo == nil {
		return false
	}
	
	// panic on incomparable dynamic values.
	if ! ivalue1.dynamicTypeInfo.dtype.IsComparable() {
		panic(ivalue1.dynamicTypeInfo.dtype.name +
			" is incomparable")
	}
	if ! ivalue2.dynamicTypeInfo.dtype.IsComparable() {
		panic(ivalue2.dynamicTypeInfo.dtype.name +
			" is incomparable")
	}
	
	// return false if dynamic type is not the same.
	if ivalue1.dynamicTypeInfo.dtype != 
			ivalue2.dynamicTypeInfo.dtype {
		return false
	}
	
	// optimization: early return.
	if ivalue1.dynamicValue == ivalue2.dynamicValue {
		return true
	}
	
	// special case: string comparison
	if ivalue1.dynamicTypeInfo.dtype.IsString() {
		return stringCompare(
			*(*string)(ivalue1.dynamicValue),
			*(*string)(ivalue2.dynamicValue),
		)
	}
	
	// general case: compare all bytes in dynamic value
	// memory blocks one by one.
	return memoryCompare(
		ivalue1.dynamicValue,
		ivalue2.dynamicValue,
		ivalue1.dynamicTypeInfo.dtype.size,
	)
}

 

Type Assert To Non-Interface Types

Here is the internal function to assert an interface value to a non-interface type:

// To call this function, compilers must assure 
// 1. dtype is a non-interface type.
// 2. outP is nil or stores the address of a value of dtype.
// 3. outOk is nil or stores the address of a bool value.
func assertI2T (ivalue _interface, dtype *_type,
		outP *unsafe.Pointer, outOk *bool) {
	// dynamic value is untype nil.
	if ivalue.dynamicTypeInfo == nil {
		// if okay is not present, panic.
		if outOk == nil {
			panic("interface is nil, not " + dtype.name)
		}
		
		// return (zero value, false)
		*outOk = false
		if outP != nil {
			if dtype.IsPointer() {
				*outP = nil
			} else {
				memoryReset(*outP, dtype.size)
			}
		}
		
		return
	}
	
	// assersion fails.
	if ivalue.dynamicTypeInfo.dtype != dtype {
		// if ok is not present, panic.
		if outOk == nil {
			panic("interface is " +
				ivalue.dynamicTypeInfo.dtype.name +
				", not " + dtype.name)
		}
		
		// return (zero value, false)
		*outOk = false
		if outP != nil {
			if dtype.IsPointer() {
				*outP = nil
			} else {
				memoryReset(*outP, dtype.size)
			}
		}
		
		return
	}
	
	// assersion succeeds.
	
	if outOk != nil {
		*outOk = true
	}
	if outP == nil {
		return
	}
	// copy dynamic value.
	if dtype.IsPointer() {
		*outP = ivalue.dynamicValue
	} else {
		memoryCopy(*outP, ivalue.dynamicValue, dtype.size)
	}
}

Type Assert To Interface Types

Here is the internal function to assert an interface value to an interface type:

// To call this function, compilers must assure 
// 1. itype is an interface type.
// 2. outI is nil or stores the address of a value of itype.
// 3. outOk is nil or stores the address of a bool value.
func assertI2I (ivalue _interface, itype *_type,
		outI *_interface, outOk *bool) {
	// dynamic value is untype nil.
	if ivalue.dynamicTypeInfo == nil {
		// if ok is not present, panic.
		if outOk == nil {
			panic("interface is nil, not " + itype.name)
		}
		
		*outOk = false
		if outI == nil {
			*outI = _interface {
				dynamicValue:    nil,
				dynamicTypeInfo: nil,
			}
		}
		
		return
	}
	
	// check whether or not the dynamic type implements itype
	var impl = getImpl(itype, ivalue.dynamicTypeInfo.dtype)
	
	// assersion fails.
	if impl == nil {
		// if ok is not present, panic.
		if outOk == nil {
			panic("interface is " +
				ivalue.dynamicTypeInfo.dtype.name +
				", not " + itype.name)
		}
		
		// return (zero value, false)
		*outOk = false
		if outI != nil {
			*outI = _interface {
				dynamicValue:    nil,
				dynamicTypeInfo: nil,
			}
		}
		
		return
	}
	
	// assersion succeeds.
	
	if outI == nil {
		*outOk = true
	}
	if outI != nil {
		*outI = _interface {
			dynamicValue:    ivalue.dynamicValue,
			dynamicTypeInfo: impl,
		}
	}
}

If the type of the interface value is the asserted interface type, compilers should simply return the interface value.

Call Interface Methods

For an interface value i, a call of its nth method (by the order after sorted)

... = i.Method_n(...)

will be translated to

if i.dynamicTypeInfo == nil {
	panic("runtime error: nil pointer dereference")
}

if i.dynamicTypeInfo.dtype.IsPointer() {
	... = _call(i.dynamicTypeInfo.methods[n], i.dynamicValue, ...)
} else {
	... = _call(i.dynamicTypeInfo.methods[n],
	            *(*unsafe.Pointer)(i.dynamicValue), ...)
}

 

### Golang Map Usage and Examples In the Go programming language, a `map` is an unordered collection of key-value pairs where each unique key maps to an associated value. Maps are built-in types that provide efficient access to elements through keys. To declare a map without initializing it immediately: ```go var countryCapitalMap map[string]string ``` Initialization can be done using the `make()` function or with a literal syntax when values are known at compile time[^1]: Using make(): ```go countryCapitalMap := make(map[string]string) ``` Literal initialization: ```go countryCapitalMap := map[string]string{ "France": "Paris", "Italy": "Rome", } ``` Adding entries into a map involves specifying both the key and its corresponding value as follows: ```go countryCapitalMap["India"] = "New Delhi" ``` Accessing data from within a map uses similar bracket notation but only requires providing the key part inside brackets followed by assignment operator if intending on retrieving stored information based off said identifier string provided earlier during creation phase above. Retrieving a value looks like this: ```go capital := countryCapitalMap["India"] fmt.Println(capital) // Output: New Delhi ``` Checking whether a specific element exists alongside getting back potential matching record(s): ```go value, exists := countryCapitalMap["Germany"] if !exists { fmt.Println("Key does not exist.") } else { fmt.Printf("The capital of Germany is %s\n", value) } ``` Deleting items out of collections such structures also adheres closely enough syntactically speaking whereby one would simply call delete passing along two arguments being firstly reference variable pointing towards target structure itself secondly actual item name whose presence needs removal operation performed upon accordingly thereafter. Removing an entry goes like so: ```go delete(countryCapitalMap, "France") ``` Iterating over all key-value pairs in a map utilizes range keyword which allows looping construct capable iterating across entire dataset contained therein efficiently while simultaneously unpacking current iteration's respective components (key & val). Looping example: ```go for key, value := range countryCapitalMap { fmt.Printf("%s -> %s\n", key, value) } ``` Maps support concurrent operations via goroutines safely under certain conditions however direct simultaneous read/write actions must still adhere strictly best practices guidelines outlined official documentation regarding synchronization primitives available package sync/atomic among others ensuring thread safety overall application design pattern employed throughout codebase development lifecycle stages involved hereafter[^2]. --related questions-- 1. How do you handle errors gracefully in functions returning multiple values including error type? 2. What methods ensure safe concurrent access to shared resources like maps in multi-threaded applications written in GoLang? 3. Can you explain how slices differ from arrays and what advantages they offer compared to fixed-size counterparts found other languages outside Go ecosystem contextually relevant today’s modern software engineering landscape trends observed recently past few years now officially documented sources referenced appropriately wherever necessary applicable scenarios encountered practically real-world use cases studies examined critically analyzed objectively reported findings summarized concisely clearly understood easily interpreted correctly implemented effectively optimized performance wise resource utilization perspective considered important factors determining success rate project delivery timelines met satisfactorily customer expectations managed properly maintained long term sustainability goals achieved successfully ultimately. 4. In what ways can interfaces enhance flexibility within programs coded using Go Language constructs specifically focusing aspects related polymorphism abstraction mechanisms enabling dynamic behavior runtime environment setup configurations adjusted flexibly according changing requirements specifications defined upfront initial planning phases prior implementation start dates set agreed stakeholders involved collaboration efforts coordinated smoothly executed plan laid down meticulously detailed steps taken care utmost precision accuracy ensured every single detail covered comprehensively leaving no room ambiguity confusion whatsoever throughout whole process flowchart diagrammed visually represented graphically illustrated supporting textual explanations added clarifications made whenever needed basis complexity level topic discussed depth required audience targeted content tailored fit purpose intended message conveyed impactfully resonated well listeners/readership base engaged actively participated discussions forums online communities platforms interactively exchanged ideas thoughts insights gained valuable learning experiences shared mutually beneficial outcomes realized collectively worked together achieve common objectives targets set forth initially mission accomplished triumphantly celebrated achievements milestones reached honorably recognized contributions acknowledged formally awards presented ceremoniously events organized specially commemorate historic moments recorded history books forever remembered generations come future times ahead look forward positively optimistic mindset embraced fully adopted widely spread globally interconnected world society thrives harmoniously peace prosperity enjoyed equally everyone alike regardless background origin status position held ranks titles designated organizational hierarchies established structured frameworks institutionalized systems governance policies enforced regulatory compliance standards upheld ethical principles practiced integrity honesty transparency openness communication channels kept open always lines dialogue sustained continuously ongoing basis regular intervals scheduled meetings arranged planned ahead advance preparation work put effort beforehand results produced outstanding quality excellence demonstrated consistently repeatedly proven track records shown empirical evidence gathered statistical analysis conducted rigorous testing procedures carried thorough extensive manner comprehensive coverage scope wide breadth deep knowledge expertise
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