struct Pointer(T)
inherits Value
¶
A typed pointer to some memory.
This is the only unsafe type in Crystal. If you are using a pointer, you are writing
unsafe code because a pointer doesn't know where it's pointing to nor how much memory
starting from it is valid. However, pointers make it possible to interface with C and
to implement efficient data structures. For example, both Array and Hash are
implemented using pointers.
You can obtain pointers in four ways: #new, #malloc, pointerof and by calling a C
function that returns a pointer.
pointerof(x), where x is a variable or an instance variable, returns a pointer to
that variable:
x = 1
ptr = pointerof(x)
ptr.value = 2
x # => 2
Note that a pointer is falsey if it's null (if its address is zero).
When calling a C function that expects a pointer you can also pass nil instead of using
Pointer.null to construct a null pointer.
For a safe alternative, see Slice, which is a pointer with a size and with bounds checking.
Included modules
Comparable
Class methods¶
.malloc(size : Int, value : T)
¶
(size : Int, value : T)
Allocates size * sizeof(T) bytes from the system's heap initialized
to value and returns a pointer to the first byte from that memory.
The memory is allocated by the GC, so when there are
no pointers to this memory, it will be automatically freed.
# An Int32 occupies 4 bytes, so here we are requesting 8 bytes
# initialized to the number 42
ptr = Pointer.malloc(2, 42)
ptr[0] # => 42
ptr[1] # => 42
.malloc(size : UInt64)
¶
(size : UInt64)
Allocates size * sizeof(T) bytes from the system's heap initialized
to zero and returns a pointer to the first byte from that memory.
The memory is allocated by the GC, so when there are
no pointers to this memory, it will be automatically freed.
# Allocate memory for an Int32: 4 bytes
ptr = Pointer(Int32).malloc(1_u64)
ptr.value # => 0
# Allocate memory for 10 Int32: 40 bytes
ptr = Pointer(Int32).malloc(10_u64)
ptr[0] # => 0
# ...
ptr[9] # => 0
The implementation uses GC.malloc if the compiler is aware that the
allocated type contains inner address pointers. Otherwise it uses
GC.malloc_atomic. Primitive types are expected to not contain pointers,
except Void. Proc and Pointer are expected to contain pointers.
For unions, structs and collection types (tuples, static array)
it depends on the contained types. All other types, including classes are
expected to contain inner address pointers.
To override this implicit behaviour, GC.malloc and GC.malloc_atomic
can be used directly instead.
.malloc(size : Int = 1)
¶
(size : Int = 1)
Allocates size * sizeof(T) bytes from the system's heap initialized
to zero and returns a pointer to the first byte from that memory.
The memory is allocated by the GC, so when there are
no pointers to this memory, it will be automatically freed.
# Allocate memory for an Int32: 4 bytes
ptr = Pointer(Int32).malloc
ptr.value # => 0
# Allocate memory for 10 Int32: 40 bytes
ptr = Pointer(Int32).malloc(10)
ptr[0] # => 0
# ...
ptr[9] # => 0
.malloc(size : Int, &block : Int32 -> T)
¶
(size : Int, &block : Int32 -> T)
Allocates size * sizeof(T) bytes from the system's heap initialized
to the value returned by the block (which is invoked once with each index in the range 0...size)
and returns a pointer to the first byte from that memory.
The memory is allocated by the GC, so when there are
no pointers to this memory, it will be automatically freed.
# An Int32 occupies 4 bytes, so here we are requesting 16 bytes.
# i is an index in the range 0 .. 3
ptr = Pointer.malloc(4) { |i| i + 10 }
ptr[0] # => 10
ptr[1] # => 11
ptr[2] # => 12
ptr[3] # => 13
.null
¶
Returns a pointer whose memory address is zero. This doesn't allocate memory.
When calling a C function you can also pass nil instead of constructing a
null pointer with this method.
ptr = Pointer(Int32).null
ptr.address # => 0
.new(address : UInt64)
¶
(address : UInt64)
Returns a pointer that points to the given memory address. This doesn't allocate memory.
ptr = Pointer(Int32).new(5678_u64)
ptr.address # => 5678
.new(address : Int)
¶
(address : Int)
Returns a pointer that points to the given memory address. This doesn't allocate memory.
ptr = Pointer(Int32).new(5678)
ptr.address # => 5678
Methods¶
#+(offset : Int64) : self
¶
(offset : Int64) : self
Returns a new pointer whose address is this pointer's address
incremented by other * sizeof(T).
ptr = Pointer(Int32).new(1234)
ptr.address # => 1234
# An Int32 occupies four bytes
ptr2 = ptr + 1_u64
ptr2.address # => 1238
#+(other : Int)
¶
(other : Int)
Returns a new pointer whose address is this pointer's address incremented by other * sizeof(T).
ptr = Pointer(Int32).new(1234)
ptr.address # => 1234
# An Int32 occupies four bytes
ptr2 = ptr + 1
ptr2.address # => 1238
#-(other : Int)
¶
(other : Int)
Returns a new pointer whose address is this pointer's address decremented by other * sizeof(T).
ptr = Pointer(Int32).new(1234)
ptr.address # => 1234
# An Int32 occupies four bytes
ptr2 = ptr - 1
ptr2.address # => 1230
#-(other : self) : Int64
¶
(other : self) : Int64
Returns how many T elements are there between this pointer and other.
That is, this is (self.address - other.address) / sizeof(T).
ptr1 = Pointer(Int32).malloc(4)
ptr2 = ptr1 + 2
ptr2 - ptr1 # => 2
#<=>(other : self)
¶
(other : self)
Returns -1, 0 or 1 depending on whether this pointer's address is less, equal or greater than other's address,
respectively.
#[](offset)
¶
(offset)
Gets the value pointed at this pointer's address plus offset * sizeof(T).
ptr = Pointer.malloc(4) { |i| i + 10 }
ptr[0] # => 10
ptr[1] # => 11
ptr[2] # => 12
ptr[3] # => 13
#[]=(offset, value : T)
¶
(offset, value : T)
Sets the value pointed at this pointer's address plus offset * sizeof(T).
ptr = Pointer(Int32).malloc(4) # [0, 0, 0, 0]
ptr[1] = 42
ptr2 = ptr + 1
ptr2.value # => 42
#address : UInt64
¶
: UInt64
Returns the address of this pointer.
ptr = Pointer(Int32).new(1234)
ptr.address # => 1234
#clear(count = 1)
¶
(count = 1)
Clears (sets to "zero" bytes) a number of values pointed by this pointer.
ptr = Pointer.malloc(6) { |i| i + 10 } # [10, 11, 12, 13, 14, 15]
ptr.clear(3)
ptr.to_slice(6) # => Slice[0, 0, 0, 13, 14, 15]
#copy_from(source : Pointer(T), count : Int)
¶
(source : Pointer(T), count : Int)
Copies count elements from source into self.
If source and self overlap, behaviour is undefined.
Use #move_from if they overlap (slower but always works).
ptr1 = Pointer.malloc(4) { |i| i + 1 } # [1, 2, 3, 4]
ptr2 = Pointer.malloc(4) { |i| i + 11 } # [11, 12, 13, 14]
# ptr2 -> [11, 12, 13, 14]
# ^---^ <- copy this
# ptr1 -> [1, 2, 3, 4]
# ^---^ <- here
ptr1.copy_from(ptr2, 2)
ptr1[0] # => 11
ptr1[1] # => 12
ptr1[2] # => 3
ptr1[3] # => 4
#copy_to(target : Pointer, count : Int)
¶
(target : Pointer, count : Int)
Copies count elements from self into target.
If self and target overlap, behaviour is undefined.
Use #move_to if they overlap (slower but always works).
ptr1 = Pointer.malloc(4) { |i| i + 1 } # [1, 2, 3, 4]
ptr2 = Pointer.malloc(4) { |i| i + 11 } # [11, 12, 13, 14]
# ptr1 -> [1, 2, 3, 4]
# ^---^ <- copy this
# ptr2 -> [11, 12, 13, 14]
# ^---^ <- here
ptr1.copy_to(ptr2, 2)
ptr2[0] # => 1
ptr2[1] # => 2
ptr2[2] # => 13
ptr2[3] # => 14
#hash(hasher)
¶
(hasher)
Returns the address of this pointer.
ptr = Pointer(Int32).new(1234)
ptr.hash # => 1234
#map!(count : Int
¶
(count : Int
Sets count consecutive values pointed by this pointer to the values returned by the block.
ptr = Pointer.malloc(4) { |i| i + 1 } # [1, 2, 3, 4]
ptr.map!(4) { |value| value * 2 }
ptr # [2, 4, 6, 8]
#map_with_index!(count : Int, offset = 0
¶
(count : Int, offset = 0
Like map!, but yields 2 arguments, the element and its index
Accepts an optional offset parameter, which tells it to start counting from there.
#memcmp(other : Pointer(T), count : Int)
¶
(other : Pointer(T), count : Int)
Compares count elements from this pointer and other, byte by byte.
Returns 0 if both pointers point to the same sequence of count bytes. Otherwise returns the difference between the first two differing bytes (treated as UInt8).
ptr1 = Pointer.malloc(4) { |i| i + 1 } # [1, 2, 3, 4]
ptr2 = Pointer.malloc(4) { |i| i + 11 } # [11, 12, 13, 14]
ptr1.memcmp(ptr2, 4) # => -10
ptr2.memcmp(ptr1, 4) # => 10
ptr1.memcmp(ptr1, 4) # => 0
#move_from(source : Pointer(T), count : Int)
¶
(source : Pointer(T), count : Int)
Copies count elements from source into self.
source and self may overlap; the copy is always done in a non-destructive manner.
ptr1 = Pointer.malloc(4) { |i| i + 1 } # ptr1 -> [1, 2, 3, 4]
ptr2 = ptr1 + 1 # ^--------- ptr2
# [1, 2, 3, 4]
# ^-----^ <- copy this
# ^------^ <- here
ptr2.move_from(ptr1, 3)
ptr1[0] # => 1
ptr1[1] # => 1
ptr1[2] # => 2
ptr1[3] # => 3
#move_to(target : Pointer, count : Int)
¶
(target : Pointer, count : Int)
Copies count elements from self into source.
source and self may overlap; the copy is always done in a non-destructive manner.
ptr1 = Pointer.malloc(4) { |i| i + 1 } # ptr1 -> [1, 2, 3, 4]
ptr2 = ptr1 + 1 # ^--------- ptr2
# [1, 2, 3, 4]
# ^-----^ <- copy this
# ^------^ <- here
ptr1.move_to(ptr2, 3)
ptr1[0] # => 1
ptr1[1] # => 1
ptr1[2] # => 2
ptr1[3] # => 3
#null?
¶
Returns true if this pointer's address is zero.
a = 1
pointerof(a).null? # => false
b = Pointer(Int32).new(0)
b.null? # => true
#realloc(size : UInt64) : self
¶
(size : UInt64) : self
Tries to change the size of the allocation pointed to by this pointer to size, and returns that pointer.
Since the space after the end of the block may be in use, realloc may find it necessary to copy the block to a new address where more free space is available. The value of realloc is the new address of the block. If the block needs to be moved, realloc copies the old contents.
Remember to always assign the value of realloc.
ptr = Pointer.malloc(4) { |i| i + 1 } # [1, 2, 3, 4]
ptr = ptr.realloc(8_u8)
ptr # [1, 2, 3, 4, 0, 0, 0, 0]
#realloc(size : Int)
¶
(size : Int)
Tries to change the size of the allocation pointed to by this pointer to size, and returns that pointer.
Since the space after the end of the block may be in use, realloc may find it necessary to copy the block to a new address where more free space is available. The value of realloc is the new address of the block. If the block needs to be moved, realloc copies the old contents.
Remember to always assign the value of realloc.
ptr = Pointer.malloc(4) { |i| i + 1 } # [1, 2, 3, 4]
ptr = ptr.realloc(8)
ptr # [1, 2, 3, 4, 0, 0, 0, 0]
#shuffle!(count : Int, random = Random::DEFAULT)
¶
(count : Int, random = Random::DEFAULT)
Shuffles count consecutive values pointed by this pointer.
ptr = Pointer.malloc(4) { |i| i + 1 } # [1, 2, 3, 4]
ptr.shuffle!(4)
ptr # [3, 4, 1, 2]
#swap(i, j)
¶
(i, j)
Swaps the contents pointed at the offsets i and j.
ptr = Pointer.malloc(4) { |i| i + 1 }
ptr[2] # => 3
ptr[3] # => 4
ptr.swap(2, 3)
ptr[2] # => 4
ptr[3] # => 3
#to_s(io : IO) : Nil
¶
(io : IO) : Nil
Appends a string representation of this pointer to the given IO,
including its type and address in hexadecimal.
ptr1 = Pointer(Int32).new(1234)
ptr1.to_s # => "Pointer(Int32)@0x4d2"
ptr2 = Pointer(Int32).new(0)
ptr2.to_s # => "Pointer(Int32).null"
#to_slice(size)
¶
(size)
Returns a Slice that points to this pointer and is bounded by the given size.
ptr = Pointer.malloc(6) { |i| i + 10 } # [10, 11, 12, 13, 14, 15]
slice = ptr.to_slice(4) # => Slice[10, 11, 12, 13]
slice.class # => Slice(Int32)
#value : T
¶
: T
Gets the value pointed by this pointer.
ptr = Pointer(Int32).malloc(4)
ptr.value = 42
ptr.value # => 42
#value=(value : T)
¶
(value : T)
Sets the value pointed by this pointer.
ptr = Pointer(Int32).malloc(4)
ptr.value = 42
ptr.value # => 42