Could I Ever Want to Access the Address Zero

Could I ever want to access the address zero?

Neither in C nor in C++ null-pointer value is in any way tied to physical address 0. The fact that you use constant 0 in the source code to set a pointer to null-pointer value is nothing more than just a piece of syntactic sugar. The compiler is required to translate it into the actual physical address used as null-pointer value on the specific platform.

In other words, 0 in the source code has no physical importance whatsoever. It could have been 42 or 13, for example. I.e. the language authors, if they so pleased, could have made it so that you'd have to do p = 42 in order to set the pointer p to null-pointer value. Again, this does not mean that the physical address 42 would have to be reserved for null pointers. The compiler would be required to translate source code p = 42 into machine code that would stuff the actual physical null-pointer value (0x0000 or 0xBAAD) into the pointer p. That's exactly how it is now with constant 0.

Also note, that neither C nor C++ provides a strictly defined feature that would allow you to assign a specific physical address to a pointer. So your question about "how one would assign 0 address to a pointer" formally has no answer. You simply can't assign a specific address to a pointer in C/C++. However, in the realm of implementation-defined features, the explicit integer-to-pointer conversion is intended to have that effect. So, you'd do it as follows

uintptr_t address = 0;
void *p = (void *) address;

Note, that this is not the same as doing

void *p = 0;

The latter always produces the null-pointer value, while the former in general case does not. The former will normally produce a pointer to physical address 0, which might or might not be the null-pointer value on the given platform.

How do I do a literal *int64 in Go?

The Go Language Specification (Address operators) does not allow to take the address of a numeric constant (not of an untyped nor of a typed constant).

The operand must be addressable, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. As an exception to the addressability requirement, x [in the expression of &x] may also be a (possibly parenthesized) composite literal.

For reasoning why this isn't allowed, see related question: Find address of constant in go. A similar question (similarly not allowed to take its address): How can I store reference to the result of an operation in Go?

0) Generic solution (from Go 1.18)

Generics are added in Go 1.18. This means we can create a single, generic Ptr() function that returns a pointer to whatever value we pass to it. Hopefully it'll get added to the standard library. Until then, you can use github.com/icza/gog, the gog.Ptr() function (disclosure: I'm the author).

This is how it can look like:

func Ptr[T any](v T) *T {
    return &v
}

Testing it:

i := Ptr(2)
log.Printf("%T %v", i, *i)

s := Ptr("abc")
log.Printf("%T %v", s, *s)

x := Ptr[any](nil)
log.Printf("%T %v", x, *x)

Which will output (try it on the Go Playground):

2009/11/10 23:00:00 *int 2
2009/11/10 23:00:00 *string abc
2009/11/10 23:00:00 *interface {} <nil>

Your other options (prior to Go 1.18) (try all on the Go Playground):

1) With new()

You can simply use the builtin new() function to allocate a new zero-valued int64 and get its address:

instance := SomeType{
    SomeField: new(int64),
}

But note that this can only be used to allocate and obtain a pointer to the zero value of any type.

2) With helper variable

Simplest and recommended for non-zero elements is to use a helper variable whose address can be taken:

helper := int64(2)
instance2 := SomeType{
    SomeField: &helper,
}

3) With helper function

Note: Helper functions to acquire a pointer to a non-zero value are available in my github.com/icza/gox library, in the gox package, so you don't have to add these to all your projects where you need it.

Or if you need this many times, you can create a helper function which allocates and returns an *int64:

func create(x int64) *int64 {
    return &x
}

And using it:

instance3 := SomeType{
    SomeField: create(3),
}

Note that we actually didn't allocate anything, the Go compiler did that when we returned the address of the function argument. The Go compiler performs escape analysis, and allocates local variables on the heap (instead of the stack) if they may escape the function. For details, see Is returning a slice of a local array in a Go function safe?

4) With a one-liner anonymous function

instance4 := SomeType{
    SomeField: func() *int64 { i := int64(4); return &i }(),
}

Or as a (shorter) alternative:

instance4 := SomeType{
    SomeField: func(i int64) *int64 { return &i }(4),
}

5) With slice literal, indexing and taking address

If you would want *SomeField to be other than 0, then you need something addressable.

You can still do that, but that's ugly:

instance5 := SomeType{
    SomeField: &[]int64{5}[0],
}
fmt.Println(*instance2.SomeField) // Prints 5

What happens here is an []int64 slice is created with a literal, having one element (5). And it is indexed (0th element) and the address of the 0th element is taken. In the background an array of [1]int64 will also be allocated and used as the backing array for the slice. So there is a lot of boilerplate here.

6) With a helper struct literal

Let's examine the exception to the addressability requirements:

As an exception to the addressability requirement, x [in the expression of &x] may also be a (possibly parenthesized) composite literal.

This means that taking the address of a composite literal, e.g. a struct literal is ok. If we do so, we will have the struct value allocated and a pointer obtained to it. But if so, another requirement will become available to us: "field selector of an addressable struct operand". So if the struct literal contains a field of type int64, we can also take the address of that field!

Let's see this option in action. We will use this wrapper struct type:

type intwrapper struct {
    x int64
}

And now we can do:

instance6 := SomeType{
    SomeField: &(&intwrapper{6}).x,
}

Note that this

&(&intwrapper{6}).x

means the following:

& ( (&intwrapper{6}).x )

But we can omit the "outer" parenthesis as the address operator & is applied to the result of the selector expression.

Also note that in the background the following will happen (this is also a valid syntax):

&(*(&intwrapper{6})).x

7) With helper anonymous struct literal

The principle is the same as with case #6, but we can also use an anonymous struct literal, so no helper/wrapper struct type definition needed:

instance7 := SomeType{
    SomeField: &(&struct{ x int64 }{7}).x,
}

Pointers vs. values in parameters and return values

tl;dr:

• Methods using receiver pointers are common; the rule of thumb for receivers is, "If in doubt, use a pointer."
• Slices, maps, channels, strings, function values, and interface values are implemented with pointers internally, and a pointer to them is often redundant.
• Elsewhere, use pointers for big structs or structs you'll have to change, and otherwise pass values, because getting things changed by surprise via a pointer is confusing.

One case where you should often use a pointer:

• Receivers are pointers more often than other arguments. It's not unusual for methods to modify the thing they're called on, or for named types to be large structs, so the guidance is to default to pointers except in rare cases.
  
  
  • Jeff Hodges' copyfighter tool automatically searches for non-tiny receivers passed by value.
    

Some situations where you don't need pointers:

• Code review guidelines suggest passing small structs like type Point struct { latitude, longitude float64 }, and maybe even things a bit bigger, as values, unless the function you're calling needs to be able to modify them in place.

  • Value semantics avoid aliasing situations where an assignment over here changes a value over there by surprise.
  • Passing small structs by value can be more efficient by avoiding cache misses or heap allocations. In any case, when pointers and values perform similarly, the Go-y approach is to choose whatever provides the more natural semantics rather than squeeze out every last bit of speed.
  • So, Go Wiki's code review comments page suggests passing by value when structs are small and likely to stay that way.
  • If the "large" cutoff seems vague, it is; arguably many structs are in a range where either a pointer or a value is OK. As a lower bound, the code review comments suggest slices (three machine words) are reasonable to use as value receivers. As something nearer an upper bound, bytes.Replace takes 10 words' worth of args (three slices and an int). You can find situations where copying even large structs turns out a performance win, but the rule of thumb is not to.
    

• For slices, you don't need to pass a pointer to change elements of the array. io.Reader.Read(p []byte) changes the bytes of p, for instance. It's arguably a special case of "treat little structs like values," since internally you're passing around a little structure called a slice header (see Russ Cox (rsc)'s explanation). Similarly, you don't need a pointer to modify a map or communicate on a channel.

• For slices you'll reslice (change the start/length/capacity of), built-in functions like append accept a slice value and return a new one. I'd imitate that; it avoids aliasing, returning a new slice helps call attention to the fact that a new array might be allocated, and it's familiar to callers.

  • It's not always practical follow that pattern. Some tools like database interfaces or serializers need to append to a slice whose type isn't known at compile time. They sometimes accept a pointer to a slice in an interface{} parameter.

• Maps, channels, strings, and function and interface values, like slices, are internally references or structures that contain references already, so if you're just trying to avoid getting the underlying data copied, you don't need to pass pointers to them. (rsc wrote a separate post on how interface values are stored).

  • You still may need to pass pointers in the rarer case that you want to modify the caller's struct: flag.StringVar takes a *string for that reason, for example.

Where you use pointers:

• Consider whether your function should be a method on whichever struct you need a pointer to. People expect a lot of methods on x to modify x, so making the modified struct the receiver may help to minimize surprise. There are guidelines on when receivers should be pointers.

• Functions that have effects on their non-receiver params should make that clear in the godoc, or better yet, the godoc and the name (like reader.WriteTo(writer)).

• You mention accepting a pointer to avoid allocations by allowing reuse; changing APIs for the sake of memory reuse is an optimization I'd delay until it's clear the allocations have a nontrivial cost, and then I'd look for a way that doesn't force the trickier API on all users:

  1. For avoiding allocations, Go's escape analysis is your friend. You can sometimes help it avoid heap allocations by making types that can be initialized with a trivial constructor, a plain literal, or a useful zero value like bytes.Buffer.
  2. Consider a Reset() method to put an object back in a blank state, like some stdlib types offer. Users who don't care or can't save an allocation don't have to call it.
  3. Consider writing modify-in-place methods and create-from-scratch functions as matching pairs, for convenience: existingUser.LoadFromJSON(json []byte) error could be wrapped by NewUserFromJSON(json []byte) (*User, error). Again, it pushes the choice between laziness and pinching allocations to the individual caller.
  4. Callers seeking to recycle memory can let sync.Pool handle some details. If a particular allocation creates a lot of memory pressure, you're confident you know when the alloc is no longer used, and you don't have a better optimization available, sync.Pool can help. (CloudFlare published a useful (pre-sync.Pool) blog post about recycling.)

Finally, on whether your slices should be of pointers: slices of values can be useful, and save you allocations and cache misses. There can be blockers:

• The API to create your items might force pointers on you, e.g. you have to call NewFoo() *Foo rather than let Go initialize with the zero value.
• The desired lifetimes of the items might not all be the same. The whole slice is freed at once; if 99% of the items are no longer useful but you have pointers to the other 1%, all of the array remains allocated.
• Copying or moving the values might cause you performance or correctness problems, making pointers more attractive. Notably, append copies items when it grows the underlying array. Pointers to slice items from before the append may not point to where the item was copied after, copying can be slower for huge structs, and for e.g. sync.Mutex copying isn't allowed. Insert/delete in the middle and sorting also move items around so similar considerations can apply.

Broadly, value slices can make sense if either you get all of your items in place up front and don't move them (e.g., no more appends after initial setup), or if you do keep moving them around but you're confident that's OK (no/careful use of pointers to items, and items are small or you've measured the perf impact). Sometimes it comes down to something more specific to your situation, but that's a rough guide.

Is NULL always zero in C?

I'm assuming you mean the null pointer. It is guaranteed to compare equal to 0.¹ But it doesn't have to be represented with all-zero bits.²

See also the comp.lang.c FAQ on null pointers.

var http=require('http');

var server=http.createServer(function(req,res){
    res.end('test');
});

server.on('listening',function(){
    console.log('ok, server is running');
});

server.listen(80);

myString := "Hi"
fmt.Println(*&myString)  // prints "Hi"

myStructPointer = &myStruct
// ...
(*myStructPointer).someAttribute = "New Value"

var str_pointer *string