Pointers in Go
External
Internal
TODO
Further reading:
- https://medium.com/@meeusdylan/when-to-use-pointers-in-go-44c15fe04eac
- https://www.ardanlabs.com/blog/2013/07/understanding-pointers-and-memory.html
- https://go.dev/doc/faq#Pointers
Overview
A pointer is a data type that represents a virtual address in memory, usually the address of a location in memory that is referred by a variable.
A pointer can be declared as such:
var aPtr *int // a pointer to an int
It can also be implicitly declared using the short variable declaration and the the referencing operator inside functions:
a := 10
aPtr := &a
aPtr is a pointer that contains the memory address that points to the memory location associated with the variable a. Changing the memory value using a syntax that involves the pointer will surface in the value of the variable:
*aPtr = 20
println(a) // will display 20
Once a value is assigned to a pointer, with the exception of nil, Go guarantees that the thing being posted to will continue to be valid for the life time of the pointer. This allows for a pattern when what looks like a stack variable can be allocated inside a function, and a pointer to it returned outside the function. The pointer will remain valid even if the stack is unwound, the compiler will arrange for the memory location holding the value of i to be valid after the function returns:
func makeInt() *int {
i := 10
return &i
}
The pointer data type comes with two operators: & (the referencing operator), and * (the dereferencing operator).
Displaying Pointers
To display the value at memory address stored in the pointer, must dereference:
fmt.Printf("%d\n", *aPtr)
To display the memory address stored in the pointer in a hexadecimal notation, with the "0x" prefix, use %p or %v, they are equivalent for pointers:
fmt.Printf("%p\n", aPtr)
fmt.Printf("%v\n", aPtr) // same thing
This will print:
0xc000012080
For more details on the pointer, including the type of the data it points to, use:
fmt.Printf("%#v\n", aPtr)
This will print:
(*int)(0xc000012080)
Pointers can be also represented using the "%X" format specifier, which displays the pointer in base 16, upper case characters, without the "0x" prefix:
fmt.Printf("%X\n", aPtr)
This will print:
C000094018
Pointer Variable Name
Do we use someNamePtr or someName?
Also see:
The Referencing Operator &
The referencing operator (the ampersand operator) returns an address, also known as a "reference", from a variable. & should be read as "address of ...". It works with variables and also with literals. The syntax &user{name:"Bill"} where user is a struct is legal. The address is represented internally as an instance of type pointer. The address points to the location in memory where the instance associated with the "referenced" variable is stored.
&<variable-name>
color := "blue"
pointerToColor := &color
println(pointerToColor) // prints "0xc000058720"
The Dereferencing Operator *
The dereferencing operator (star operator) takes a pointer and returns the value in memory the pointer's address points toward. The variable must contain a pointer type instance, otherwise the code will not compile. The value thus exposed can be read or written.
*<pointer-name>
color := "blue"
pointerToColor := &color
println(*pointerToColor) // prints "blue"
*pointerToColor = "red"
println(color) // prints "red"