class Gem::Version

The Version class processes string versions into comparable values. A version string should normally be a series of numbers separated by periods. Each part (digits separated by periods) is considered its own number, and these are used for sorting. So for instance, 3.10 sorts higher than 3.2 because ten is greater than two.

If any part contains letters (currently only a-z are supported) then that version is considered prerelease. Versions with a prerelease part in the Nth part sort less than versions with N-1 parts. Prerelease parts are sorted alphabetically using the normal Ruby string sorting rules. If a prerelease part contains both letters and numbers, it will be broken into multiple parts to provide expected sort behavior (1.0.a10 becomes 1.0.a.10, and is greater than 1.0.a9).

Prereleases sort between real releases (newest to oldest):

  1. 1.0

  2. 1.0.b1

  3. 1.0.a.2

  4. 0.9

If you want to specify a version restriction that includes both prereleases and regular releases of the 1.x series this is the best way:

s.add_dependency 'example', '>= 1.0.0.a', '< 2.0.0'

How Software Changes

Users expect to be able to specify a version constraint that gives them some reasonable expectation that new versions of a library will work with their software if the version constraint is true, and not work with their software if the version constraint is false. In other words, the perfect system will accept all compatible versions of the library and reject all incompatible versions.

Libraries change in 3 ways (well, more than 3, but stay focused here!).

  1. The change may be an implementation detail only and have no effect on the client software.

  2. The change may add new features, but do so in a way that client software written to an earlier version is still compatible.

  3. The change may change the public interface of the library in such a way that old software is no longer compatible.

Some examples are appropriate at this point. Suppose I have a Stack class that supports a push and a pop method.

Examples of Category 1 changes:

  • Switch from an array based implementation to a linked-list based implementation.

  • Provide an automatic (and transparent) backing store for large stacks.

Examples of Category 2 changes might be:

  • Add a depth method to return the current depth of the stack.

  • Add a top method that returns the current top of stack (without changing the stack).

  • Change push so that it returns the item pushed (previously it had no usable return value).

Examples of Category 3 changes might be:

  • Changes pop so that it no longer returns a value (you must use top to get the top of the stack).

  • Rename the methods to push_item and pop_item.

RubyGems Rational Versioning

  • Versions shall be represented by three non-negative integers, separated by periods (e.g. 3.1.4). The first integers is the “major” version number, the second integer is the “minor” version number, and the third integer is the “build” number.

  • A category 1 change (implementation detail) will increment the build number.

  • A category 2 change (backwards compatible) will increment the minor version number and reset the build number.

  • A category 3 change (incompatible) will increment the major build number and reset the minor and build numbers.

  • Any “public” release of a gem should have a different version. Normally that means incrementing the build number. This means a developer can generate builds all day long, but as soon as they make a public release, the version must be updated.


Let’s work through a project lifecycle using our Stack example from above.

Version 0.0.1

The initial Stack class is release.

Version 0.0.2

Switched to a linked=list implementation because it is cooler.

Version 0.1.0

Added a depth method.

Version 1.0.0

Added top and made pop return nil (pop used to return the old top item).

Version 1.1.0

push now returns the value pushed (it used it return nil).

Version 1.1.1

Fixed a bug in the linked list implementation.

Version 1.1.2

Fixed a bug introduced in the last fix.

Client A needs a stack with basic push/pop capability. They write to the original interface (no top), so their version constraint looks like:

gem 'stack', '>= 0.0'

Essentially, any version is OK with Client A. An incompatible change to the library will cause them grief, but they are willing to take the chance (we call Client A optimistic).

Client B is just like Client A except for two things: (1) They use the depth method and (2) they are worried about future incompatibilities, so they write their version constraint like this:

gem 'stack', '~> 0.1'

The depth method was introduced in version 0.1.0, so that version or anything later is fine, as long as the version stays below version 1.0 where incompatibilities are introduced. We call Client B pessimistic because they are worried about incompatible future changes (it is OK to be pessimistic!).

Preventing Version Catastrophe:


Let’s say you’re depending on the fnord gem version 2.y.z. If you specify your dependency as “>= 2.0.0” then, you’re good, right? What happens if fnord 3.0 comes out and it isn’t backwards compatible with 2.y.z? Your stuff will break as a result of using “>=”. The better route is to specify your dependency with an “approximate” version specifier (“~>”). They’re a tad confusing, so here is how the dependency specifiers work:

Specification From  ... To (exclusive)
">= 3.0"      3.0   ... &infin;
"~> 3.0"      3.0   ... 4.0
"~> 3.0.0"    3.0.0 ... 3.1
"~> 3.5"      3.5   ... 4.0
"~> 3.5.0"    3.5.0 ... 3.6
"~> 3"        3.0   ... 4.0

For the last example, single-digit versions are automatically extended with a zero to give a sensible result.

Public Class Methods

correct?(version) click to toggle source

True if the version string matches RubyGems’ requirements.

# File rubygems/version.rb, line 173
def self.correct?(version)
  nil_versions_are_discouraged! if version.nil?

create(input) click to toggle source

Factory method to create a Version object. Input may be a Version or a String. Intended to simplify client code.

ver1 = Version.create('1.3.17')   # -> (Version object)
ver2 = Version.create(ver1)       # -> (ver1)
ver3 = Version.create(nil)        # -> nil
# File rubygems/version.rb, line 187
def self.create(input)
  if self === input # check yourself before you wreck yourself
  elsif input.nil?

    new input
new(version) click to toggle source

Constructs a Version from the version string. A version string is a series of digits or ASCII letters separated by dots.

# File rubygems/version.rb, line 221
def initialize(version)
  unless self.class.correct?(version)
    raise ArgumentError, "Malformed version number string #{version}"

  # If version is an empty string convert it to 0
  version = 0 if version.is_a?(String) && /\A\s*\Z/.match?(version)

  @version = version.to_s

  # optimization to avoid allocation when given an integer, since we know
  # it's to_s won't have any spaces or dashes
  unless version.is_a?(Integer)
    @version = @version.strip
  @version = -@version
  @segments = nil

Private Class Methods

nil_versions_are_discouraged!() click to toggle source
# File rubygems/version.rb, line 209
def self.nil_versions_are_discouraged!
  unless Gem::Deprecate.skip
    warn "nil versions are discouraged and will be deprecated in Rubygems 4"

Public Instance Methods

<=>(other) click to toggle source

Compares this version with other returning -1, 0, or 1 if the other version is larger, the same, or smaller than this one. Attempts to compare to something that’s not a Gem::Version or a valid version String return nil.

# File rubygems/version.rb, line 361
def <=>(other)
  return self <=> if (String === other) && self.class.correct?(other)

  return unless Gem::Version === other
  return 0 if @version == other.version || canonical_segments == other.canonical_segments

  lhsegments = canonical_segments
  rhsegments = other.canonical_segments

  lhsize = lhsegments.size
  rhsize = rhsegments.size
  limit  = (lhsize > rhsize ? lhsize : rhsize) - 1

  i = 0

  while i <= limit
    lhs = lhsegments[i] || 0
    rhs = rhsegments[i] || 0
    i += 1

    next      if lhs == rhs
    return -1 if String  === lhs && Numeric === rhs
    return  1 if Numeric === lhs && String  === rhs

    return lhs <=> rhs

approximate_recommendation() click to toggle source

A recommended version for use with a ~> Requirement.

# File rubygems/version.rb, line 343
def approximate_recommendation
  segments = self.segments

  segments.pop    while segments.any? {|s| String === s }
  segments.pop    while segments.size > 2
  segments.push 0 while segments.size < 2

  recommendation = "~> #{segments.join(".")}"
  recommendation += ".a" if prerelease?
bump() click to toggle source

Return a new version object where the next to the last revision number is one greater (e.g., 5.3.1 => 5.4).

Pre-release (alpha) parts, e.g, 5.3.1.b.2 => 5.4, are ignored.

# File rubygems/version.rb, line 247
def bump
  @@bump[self] ||= begin
                     segments = self.segments
                     segments.pop while segments.any? {|s| String === s }
                     segments.pop if segments.size > 1

                     segments[-1] = segments[-1].succ
canonical_segments() click to toggle source

remove trailing zeros segments before first letter or at the end of the version

# File rubygems/version.rb, line 392
def canonical_segments
  @canonical_segments ||= begin
    # remove trailing 0 segments, using dot or letter as anchor
    # may leave a trailing dot which will be ignored by partition_segments
    canonical_version = @version.sub(/(?<=[a-zA-Z.])[.0]+\z/, "")
    # remove 0 segments before the first letter in a prerelease version
    canonical_version.sub!(/(?<=\.|\A)[0.]+(?=[a-zA-Z])/, "") if prerelease?
eql?(other) click to toggle source

A Version is only eql? to another version if it’s specified to the same precision. Version “1.0” is not the same as version “1”.

# File rubygems/version.rb, line 262
def eql?(other)
  self.class === other && @version == other.version
freeze() click to toggle source
Calls superclass method
# File rubygems/version.rb, line 403
def freeze
marshal_dump() click to toggle source

Dump only the raw version string, not the complete object. It’s a string for backwards (RubyGems 1.3.5 and earlier) compatibility.

# File rubygems/version.rb, line 282
def marshal_dump
marshal_load(array) click to toggle source

Load custom marshal format. It’s a string for backwards (RubyGems 1.3.5 and earlier) compatibility.

# File rubygems/version.rb, line 290
def marshal_load(array)
  initialize array[0]
prerelease?() click to toggle source

A version is considered a prerelease if it contains a letter.

# File rubygems/version.rb, line 311
def prerelease?
  unless instance_variable_defined? :@prerelease
    @prerelease = /[a-zA-Z]/.match?(version)
release() click to toggle source

The release for this version (e.g. 1.2.0.a -> 1.2.0). Non-prerelease versions return themselves.

# File rubygems/version.rb, line 326
def release
  @@release[self] ||= if prerelease?
    segments = self.segments
    segments.pop while segments.any? {|s| String === s } segments.join(".")
Alias for: version
version() click to toggle source

A string representation of this Version.

# File rubygems/version.rb, line 164
def version
Also aliased as: to_s

Protected Instance Methods

_segments() click to toggle source
# File rubygems/version.rb, line 412
def _segments
  # segments is lazy so it can pick up version values that come from
  # old marshaled versions, which don't go through marshal_load.
  # since this version object is cached in @@all, its @segments should be frozen
  @segments ||= partition_segments(@version)
partition_segments(ver) click to toggle source
# File rubygems/version.rb, line 419
def partition_segments(ver)
  ver.scan(/\d+|[a-z]+/i).map! do |s|
    /\A\d/.match?(s) ? s.to_i : -s