struct Pointer(T)

Overview

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
puts x #=> 2

Note that a pointer is falsey if it's null (if it's address is zero).

For a safe alternative, see Slice, which is a pointer with a length and with bounds checking.

Superclass hierarchy

Object
Value
Pointer(T)

Included Modules

Comparable(self)

Defined in:

Class Method Summary

Instance Method Summary

Class Method Detail

def self.malloc(size : Int, &block : Int32 -> T)

Allocates size * sizeof(T) bytes from the system's heap initialized to the value returned by the block 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
def self.malloc(size = 1 : Int)

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
def self.malloc(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
def self.new(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
def self.null

Returns a pointer whose memory address is zero. This doesn't allocate memory.

ptr = Pointer(Int32).null
ptr.address #=> 0

Instance Method Detail

def +(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
def -(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
def <=>(other : self)

Returns -1, 0 or 1 if this pointer's address is less, equal or greater than other's address, respectively.

See Object#<=>.

def [](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
def []=(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
def appender

Returns a PointerAppender for this pointer.

def clear(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                                    #   [0, 0, 0, 13, 14, 15]
def copy_from(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
def copy_to(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
def hash

Returns the address of this pointer.

ptr = Pointer(Int32).new(1234)
ptr.hash #=> 1234
def map!(count : Int, &block)

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]
def memcmp(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
def move_from(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
def move_to(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
def nil?

Returns true if this pointer's address is zero.

a = 1
pointerof(a).nil?         #=> false

b = Pointer(Int32).new(0)
b.nil?                    #=> true
def null?

Alias of #nil?

def realloc(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]
def shuffle!(count : Int)

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]
def swap(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
def to_s(io : IO)

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
def to_slice(length)

Returns a Slice that points to this pointer and is bounded by the given length.

ptr = Pointer.malloc(6) { |i| i + 10 } #   [10, 11, 12, 13, 14, 15]
slice = ptr.to_slice(4)                #=> [10, 11, 12, 13]
slice.class                            #=> Slice(Int32)