Some Simple Summaries


Types whose values may have indirect underlying parts

Types whose values may have indirect underlying parts:
  • string types
  • function types
  • slice types
  • map types
  • channel types
  • interface types

The answer is based on the implementations of the standard Go compiler/runtime. In fact, whether or not function values may have indirect underlying parts is hardly to prove, and string values and interface values should be viewed as values without indirect underlying parts in logic. Please read value parts for details.

Types which values can be used as arguments of built-in len function (and cap, close, delete, make functions)

len cap close delete make
string Yes
(and array pointer)
Yes Yes
slice Yes Yes Yes
map Yes Yes Yes
channel Yes Yes Yes Yes

Values of above types can also be ranged over in for-range loops.

Types which values can be used as arguments of built-in function len can be called container types in broad sense.

Comparison of built-in container types

Type Can New Elements Be Added into Values? Are Elements of Values Replaceable? Are Elements of Values Addressable? Will Element Accesses Modify Value Lengths? May Values Have Underlying Parts
string No No No No Yes(1)
array No Yes(2) Yes(2) No No
slice No(3) Yes Yes No Yes
map Yes Yes No No Yes
channel Yes(4) No No Yes Yes

(1) For the standard Go compiler/runtime.
(2) For addressable array values only.
(3) Generally, a slice value are modified by assigned another slice value to it by overwriting it. Here, such cases are not viewed as "add new elements". In fact, slice lengths can also be modified separately by calling the reflect.SetLen function. Increase the length of a slice by this way is kind of adding new elements into the slice. But the reflect.SetLen function is slow, so it is rarely used.
(4) For buffered channels which are still not full.

Types which values can be represented with composite literals (T{...})

Values of the following four kinds of types can be represented with composite literals:

Type (T) Is T{} a Zero Value of T?
struct Yes
array Yes
slice No
(zero value is nil)
map No
(zero value is nil)

Value sizes of all kinds of types

Please read value copy cost for details.

Types which zero values can be represented with nil

The zero values of the following types can be represented with nil.

Type (T) Size of T(nil)
pointer 1 word
slice 3 words
map 1 word
channel 1 word
function 1 word
interface 2 words

The above listed sizes are for the standard Go compiler. One word means 4 bytes on 32-bit architectures and 8 bytes on 64-bit architectures. and the indirect underlying parts of a value don't contribute to the size of the value.

The size of a zero value of a type is the same as any other values of the same type.

Types we can implement methods for

Please read methods in Go for details.

Types which can be embedded in struct types

Please read which types can be embedded for details.

Functions whose calls will/may be evaluated at compile time

If a function call is evaluated at compile time, its return results must be constants.

Function Return Type Are Calls Always Evaluated at Compile Time?
unsafe.Sizeof uintptr Yes, always.
len int Not always.

From Go specification:
  • the expression len(s) is constant if s is a string constant.
  • the expressions len(s) and cap(s) are constants if the type of s is an array or pointer to an array and the expression s does not contain channel receives or (non-constant) function calls.
real The result is an untyped value. Its default type is float64. Not always.

From Go spec: the expressions real(s) and imag(s) are constants if s is a complex constant.
complex The result is an untyped value. Its default type is complex128. Not always.

From Go spec: the expression complex(sr, si) is constant if both sr and si are numeric constants.

Addressable and unaddressable values

Please read this FAQ item to get which values are addressable or unaddressable.

Types which don't support comparisons

Please read this FAQ item to get which values are addressable or unaddressable.

Which code elements are allowed to be declared but not used

Allowed to Be Declared but Not Used?
import No
type Yes
variable Yes for package-level variables.
No for local variables (for the standard compiler).
constant Yes
function Yes
label No

Named source code elements which can be declared together within ()

Following source code elements (of the same kind) can be declared together within ():
  • import
  • type
  • variable
  • constant

Functions can't be declared together within (). Also labels.

Named source code elements which can be declared both inside functions and outside any functions

Following named source code elements can be declared both inside functions and outside any functions:
  • type
  • variable
  • constant

Imports must be declared before declarations of other elements (and after the package clause).

Functions can only be declared outside any functions. Anonymous functions can be defined inside other function bodies, but such definitions are not function declarations.

Labels must be declared inside functions.

Expressions which evaluation results may contain optional additional values

The evaluation results of the following expressions may contain optional additional values:

Syntax Meaning of The Optional Value (ok in the syntax examples) Will Omitting the Optional Return Affect Program Behavior?
map element access e, ok = aMap[key] whether or not the accessed key is present in the map No
channel value receive e, ok = <- aChannel whether or not the received value was sent before the channel was closed No
type assertion v, ok = anInterface.(T) whether or not the dynamic type of the interface value matches the asserted type Yes
(when the optional bool return is omitted, a panic is created if the asserted type is not an ok type.)

Ways to block the current goroutine ‎forever by using the channel mechanism

Without importing any package, we can use the following ways to make the current goroutine ‎enter (and stay in) blocking state forever:
  1. send a value to a channel which no ones will receive values from
    make(chan struct{}) <- struct{}{}
    // or
    make(chan<- struct{}) <- struct{}{}
  2. receive a value from a never-closed channel which no values have been and will be sent to
    <-make(chan struct{})
    // or
    <-make(<-chan struct{})
    // or
    for range make(<-chan struct{}) {}
  3. receive a value from (or send a value to) a nil channel
    chan struct{}(nil) <- struct{}{}
    // or
    <-chan struct{}(nil)
    // or
    for range chan struct{}(nil) {}
  4. use a bare select block

Ways to concatenate strings

Please read strings in Go for details.

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