# Gem::Version < Object
Comparable (from ruby core)
(from ruby core)
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):
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
* Provide an automatic (and transparent) backing store for large
### 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
* 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
: The initial Stack class is release.
: Switched to a linked=list implementation because it is cooler.
: Added a `depth` method.
: Added `top` and made `pop` return nil (`pop` used to return the old
: `push` now returns the value pushed (it used it return nil).
: Fixed a bug in the linked list implementation.
: 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
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
## 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
Specification From ... To (exclusive)
">= 3.0" 3.0 ... ∞
"~> 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.
# Class methods:
# Instance methods: