class Symbol

A Symbol object represents a named identifier inside the Ruby interpreter.

You can create a Symbol object explicitly with:

A symbol literal.

The same Symbol object will be created for a given name or string for the duration of a program’s execution, regardless of the context or meaning of that name. Thus if Fred is a constant in one context, a method in another, and a class in a third, the Symbol :Fred will be the same object in all three contexts.

module One
  class Fred
  end
  $f1 = :Fred
end
module Two
  Fred = 1
  $f2 = :Fred
end
def Fred()
end
$f3 = :Fred
$f1.object_id   #=> 2514190
$f2.object_id   #=> 2514190
$f3.object_id   #=> 2514190

Constant, method, and variable names are returned as symbols:

module One
  Two = 2
  def three; 3 end
  @four = 4
  @@five = 5
  $six = 6
end
seven = 7

One.constants
# => [:Two]
One.instance_methods(true)
# => [:three]
One.instance_variables
# => [:@four]
One.class_variables
# => [:@@five]
global_variables.grep(/six/)
# => [:$six]
local_variables
# => [:seven]

A Symbol object differs from a String object in that a Symbol object represents an identifier, while a String object represents text or data.

What’s Here¶ ↑

First, what’s elsewhere. Class Symbol:

Inherits from class Object.
Includes module Comparable.

Here, class Symbol provides methods that are useful for:

Methods for Querying¶ ↑

::all_symbols: Returns an array of the symbols currently in Ruby’s symbol table.
=~: Returns the index of the first substring in symbol that matches a given Regexp or other object; returns nil if no match is found.
[], slice : Returns a substring of symbol determined by a given index, start/length, or range, or string.
empty?: Returns true if self.length is zero; false otherwise.
encoding: Returns the Encoding object that represents the encoding of symbol.
end_with?: Returns true if symbol ends with any of the given strings.
match: Returns a MatchData object if symbol matches a given Regexp; nil otherwise.
match?: Returns true if symbol matches a given Regexp; false otherwise.
length, size: Returns the number of characters in symbol.
start_with?: Returns true if symbol starts with any of the given strings.

Methods for Comparing¶ ↑

<=>: Returns -1, 0, or 1 as a given symbol is smaller than, equal to, or larger than symbol.
==, ===: Returns true if a given symbol has the same content and encoding.
casecmp: Ignoring case, returns -1, 0, or 1 as a given symbol is smaller than, equal to, or larger than symbol.
casecmp?: Returns true if symbol is equal to a given symbol after Unicode case folding; false otherwise.

Methods for Converting¶ ↑

capitalize: Returns symbol with the first character upcased and all other characters downcased.
downcase: Returns symbol with all characters downcased.
inspect: Returns the string representation of self as a symbol literal.
name: Returns the frozen string corresponding to symbol.
succ, next: Returns the symbol that is the successor to symbol.
swapcase: Returns symbol with all upcase characters downcased and all downcase characters upcased.
to_proc: Returns a Proc object which responds to the method named by symbol.
to_s, id2name: Returns the string corresponding to self.
to_sym, intern: Returns self.
upcase: Returns symbol with all characters upcased.

Public Class Methods

all_symbols → array_of_symbols

Returns an array of all symbols currently in Ruby’s symbol table:

Symbol.all_symbols.size    # => 9334
Symbol.all_symbols.take(3) # => [:!, :"\"", :"#"]

static VALUE
sym_all_symbols(VALUE _)
{
    return rb_sym_all_symbols();
}

json_create (o)

See as_json.

# File ext/json/lib/json/add/symbol.rb, line 45
def self.json_create(o)
  o['s'].to_sym
end

Public Instance Methods

symbol <=> object → -1, 0, +1, or nil

If object is a symbol, returns the equivalent of symbol.to_s <=> object.to_s:

:bar <=> :foo # => -1
:foo <=> :foo # => 0
:foo <=> :bar # => 1

Otherwise, returns nil:

:foo <=> 'bar' # => nil

Related: String#<=>.

static VALUE
sym_cmp(VALUE sym, VALUE other)
{
    if (!SYMBOL_P(other)) {
        return Qnil;
    }
    return rb_str_cmp_m(rb_sym2str(sym), rb_sym2str(other));
}

symbol == object → true or false

Returns true if object is the same object as self, false otherwise.

#define sym_equal rb_obj_equal

Also aliased as: ===

===

Alias for: ==

symbol =~ object → integer or nil

Equivalent to symbol.to_s =~ object, including possible updates to global variables; see String#=~.

static VALUE
sym_match(VALUE sym, VALUE other)
{
    return rb_str_match(rb_sym2str(sym), other);
}

symbol[index] → string or nil

symbol[start, length] → string or nil

symbol[range] → string or nil

symbol[regexp, capture = 0] → string or nil

symbol[substring] → string or nil

Equivalent to symbol.to_s[]; see String#[].

static VALUE
sym_aref(int argc, VALUE *argv, VALUE sym)
{
    return rb_str_aref_m(argc, argv, rb_sym2str(sym));
}

Also aliased as: slice

as_json (*)

Methods Symbol#as_json and Symbol.json_create may be used to serialize and deserialize a Symbol object; see Marshal.

Method Symbol#as_json serializes self, returning a 2-element hash representing self:

require 'json/add/symbol'
x = :foo.as_json
# => {"json_class"=>"Symbol", "s"=>"foo"}

Method JSON.create deserializes such a hash, returning a Symbol object:

Symbol.json_create(x) # => :foo

# File ext/json/lib/json/add/symbol.rb, line 24
def as_json(*)
  {
    JSON.create_id => self.class.name,
    's'            => to_s,
  }
end

capitalize(*options) → symbol

Equivalent to sym.to_s.capitalize.to_sym.

See String#capitalize.

static VALUE
sym_capitalize(int argc, VALUE *argv, VALUE sym)
{
    return rb_str_intern(rb_str_capitalize(argc, argv, rb_sym2str(sym)));
}

casecmp(object) → -1, 0, 1, or nil

Like Symbol#<=>, but case-insensitive; equivalent to self.to_s.casecmp(object.to_s):

lower = :abc
upper = :ABC
upper.casecmp(lower) # => 0
lower.casecmp(lower) # => 0
lower.casecmp(upper) # => 0

Returns nil if self and object have incompatible encodings, or if object is not a symbol:

sym = 'äöü'.encode("ISO-8859-1").to_sym
other_sym = 'ÄÖÜ'
sym.casecmp(other_sym) # => nil
:foo.casecmp(2)        # => nil

Unlike Symbol#casecmp?, case-insensitivity does not work for characters outside of ‘A’..‘Z’ and ‘a’..‘z’:

lower = :äöü
upper = :ÄÖÜ
upper.casecmp(lower) # => -1
lower.casecmp(lower) # => 0
lower.casecmp(upper) # => 1

static VALUE
sym_casecmp(VALUE sym, VALUE other)
{
    if (!SYMBOL_P(other)) {
        return Qnil;
    }
    return str_casecmp(rb_sym2str(sym), rb_sym2str(other));
}

casecmp?(object) → true, false, or nil

Returns true if self and object are equal after Unicode case folding, otherwise false:

lower = :abc
upper = :ABC
upper.casecmp?(lower) # => true
lower.casecmp?(lower) # => true
lower.casecmp?(upper) # => true

Returns nil if self and object have incompatible encodings, or if object is not a symbol:

sym = 'äöü'.encode("ISO-8859-1").to_sym
other_sym = 'ÄÖÜ'
sym.casecmp?(other_sym) # => nil
:foo.casecmp?(2)        # => nil

Unlike Symbol#casecmp, works for characters outside of ‘A’..‘Z’ and ‘a’..‘z’:

lower = :äöü
upper = :ÄÖÜ
upper.casecmp?(lower) # => true
lower.casecmp?(lower) # => true
lower.casecmp?(upper) # => true

static VALUE
sym_casecmp_p(VALUE sym, VALUE other)
{
    if (!SYMBOL_P(other)) {
        return Qnil;
    }
    return str_casecmp_p(rb_sym2str(sym), rb_sym2str(other));
}

downcase(*options) → symbol

Equivalent to sym.to_s.downcase.to_sym.

See String#downcase.

Related: Symbol#upcase.

static VALUE
sym_downcase(int argc, VALUE *argv, VALUE sym)
{
    return rb_str_intern(rb_str_downcase(argc, argv, rb_sym2str(sym)));
}

empty? → true or false

Returns true if self is :'', false otherwise.

static VALUE
sym_empty(VALUE sym)
{
    return rb_str_empty(rb_sym2str(sym));
}

encoding → encoding

Equivalent to self.to_s.encoding; see String#encoding.

static VALUE
sym_encoding(VALUE sym)
{
    return rb_obj_encoding(rb_sym2str(sym));
}

end_with?(*strings) → true or false

Equivalent to self.to_s.end_with?; see String#end_with?.

static VALUE
sym_end_with(int argc, VALUE *argv, VALUE sym)
{
    return rb_str_end_with(argc, argv, rb_sym2str(sym));
}

id2name

Alias for: to_s

inspect → string

Returns a string representation of self (including the leading colon):

:foo.inspect # => ":foo"

Related: Symbol#to_s, Symbol#name.

static VALUE
sym_inspect(VALUE sym)
{
    VALUE str = rb_sym2str(sym);
    const char *ptr;
    long len;
    char *dest;

    if (!rb_str_symname_p(str)) {
        str = rb_str_inspect(str);
        len = RSTRING_LEN(str);
        rb_str_resize(str, len + 1);
        dest = RSTRING_PTR(str);
        memmove(dest + 1, dest, len);
    }
    else {
        rb_encoding *enc = STR_ENC_GET(str);
        VALUE orig_str = str;

        len = RSTRING_LEN(orig_str);
        str = rb_enc_str_new(0, len + 1, enc);

        // Get data pointer after allocation
        ptr = RSTRING_PTR(orig_str);
        dest = RSTRING_PTR(str);
        memcpy(dest + 1, ptr, len);

        RB_GC_GUARD(orig_str);
    }
    dest[0] = ':';

    RUBY_ASSERT_BUILTIN_TYPE(str, T_STRING);

    return str;
}

intern

Alias for: to_sym

length → integer

Equivalent to self.to_s.length; see String#length.

static VALUE
sym_length(VALUE sym)
{
    return rb_str_length(rb_sym2str(sym));
}

Also aliased as: size

match(pattern, offset = 0) → matchdata or nil

match(pattern, offset = 0) {|matchdata| } → object

Equivalent to self.to_s.match, including possible updates to global variables; see String#match.

static VALUE
sym_match_m(int argc, VALUE *argv, VALUE sym)
{
    return rb_str_match_m(argc, argv, rb_sym2str(sym));
}

match?(pattern, offset) → true or false

Equivalent to sym.to_s.match?; see String#match.

static VALUE
sym_match_m_p(int argc, VALUE *argv, VALUE sym)
{
    return rb_str_match_m_p(argc, argv, sym);
}

name → string

Returns a frozen string representation of self (not including the leading colon):

:foo.name         # => "foo"
:foo.name.frozen? # => true

VALUE
rb_sym2str(VALUE sym)
{
    VALUE str;
    if (DYNAMIC_SYM_P(sym)) {
        str = RSYMBOL(sym)->fstr;
        RUBY_ASSERT_BUILTIN_TYPE(str, T_STRING);
    }
    else {
        str = rb_id2str(STATIC_SYM2ID(sym));
        if (str) RUBY_ASSERT_BUILTIN_TYPE(str, T_STRING);
    }

    return str;
}

succ

Equivalent to self.to_s.succ.to_sym:

:foo.succ # => :fop

Related: String#succ.

Alias for: succ

size

Alias for: length

slice

Alias for: []

start_with?(*string_or_regexp) → true or false

Equivalent to self.to_s.start_with?; see String#start_with?.

static VALUE
sym_start_with(int argc, VALUE *argv, VALUE sym)
{
    return rb_str_start_with(argc, argv, rb_sym2str(sym));
}

succ

Equivalent to self.to_s.succ.to_sym:

:foo.succ # => :fop

Related: String#succ.

static VALUE
sym_succ(VALUE sym)
{
    return rb_str_intern(rb_str_succ(rb_sym2str(sym)));
}

Also aliased as: next

swapcase(*options) → symbol

Equivalent to sym.to_s.swapcase.to_sym.

See String#swapcase.

static VALUE
sym_swapcase(int argc, VALUE *argv, VALUE sym)
{
    return rb_str_intern(rb_str_swapcase(argc, argv, rb_sym2str(sym)));
}

to_json (*a)

Returns a JSON string representing self:

require 'json/add/symbol'
puts :foo.to_json

Output:

# {"json_class":"Symbol","s":"foo"}

# File ext/json/lib/json/add/symbol.rb, line 40
def to_json(*a)
  as_json.to_json(*a)
end

to_proc

Returns a Proc object which calls the method with name of self on the first parameter and passes the remaining parameters to the method.

proc = :to_s.to_proc   # => #<Proc:0x000001afe0e48680(&:to_s) (lambda)>
proc.call(1000)        # => "1000"
proc.call(1000, 16)    # => "3e8"
(1..3).collect(&:to_s) # => ["1", "2", "3"]

VALUE
rb_sym_to_proc(VALUE sym)
{
    static VALUE sym_proc_cache = Qfalse;
    enum {SYM_PROC_CACHE_SIZE = 67};
    VALUE proc;
    long index;
    ID id;

    if (!sym_proc_cache) {
        sym_proc_cache = rb_ary_hidden_new(SYM_PROC_CACHE_SIZE * 2);
        rb_vm_register_global_object(sym_proc_cache);
        rb_ary_store(sym_proc_cache, SYM_PROC_CACHE_SIZE*2 - 1, Qnil);
    }

    id = SYM2ID(sym);
    index = (id % SYM_PROC_CACHE_SIZE) << 1;

    if (RARRAY_AREF(sym_proc_cache, index) == sym) {
        return RARRAY_AREF(sym_proc_cache, index + 1);
    }
    else {
        proc = sym_proc_new(rb_cProc, ID2SYM(id));
        RARRAY_ASET(sym_proc_cache, index, sym);
        RARRAY_ASET(sym_proc_cache, index + 1, proc);
        return proc;
    }
}

to_s → string

Returns a string representation of self (not including the leading colon):

:foo.to_s # => "foo"

# File symbol.rb, line 10
def to_s
  Primitive.attr! :leaf
  Primitive.cexpr! 'rb_sym_to_s(self)'
end

Also aliased as: id2name

to_sym → self

Returns self.

Related: String#to_sym.

# File symbol.rb, line 23
def to_sym
  self
end

Also aliased as: intern

upcase(*options) → symbol

Equivalent to sym.to_s.upcase.to_sym.

See String#upcase.

static VALUE
sym_upcase(int argc, VALUE *argv, VALUE sym)
{
    return rb_str_intern(rb_str_upcase(argc, argv, rb_sym2str(sym)));
}