Class: ImmutableString

• Inheritance
• Description
• Related information
• Instance protocol
  • accessing
  • converting
  • copying
  • error handling
  • private
  • specials

Inheritance:

   Object
   |
   +--Collection
      |
      +--SequenceableCollection
         |
         +--ArrayedCollection
            |
            +--UninterpretedBytes
               |
               +--ByteArray
                  |
                  +--CharacterArray
                     |
                     +--String
                        |
                        +--ImmutableString

Package:

stx:libbasic

Category:

System-Compiler-Support

Version:

rev: 1.9 date: 2009/11/16 08:59:40

user: stefan

file: ImmutableString.st directory: libbasic

module: stx stc-classLibrary: libbasic

Author:

Claus Gittinger

Description:

By default, string literals in smalltalk are mutable objects. That
may lead to some subtle (and hard to find errors), if some method passes
a string constant as argument to someone else, who changes the
string using at:put: like messages. Since the string object is kept in
the first methods literals, the string constant has now been changed without
having the method's sourcecode reflect this. Thus, the method will
behave differently from what its source may make you think.

To help finding this kind of 'feature/bug', the compiler can be
configured to create instances of this ImmutableString instead of Strings
for literals. Instances of ImmutableString catch storing accesses and
enter the debugger. Although useful, this feature is disabled by default
for compatibility to other smalltalk implementations.
(Also, if turned on, this makes inspecting string literals entered in
 a workspace somewhat strange: you cannot modify it any longer).

Turn the ImmutableString feature on by setting the Parsers class variable
'StringsAreImmutable' to true or use the new launchers settings menu.


ATTENTION:
    there may be still code around which checks for explicit class being String
    (both in Smalltalk and in primitive code). All code like foo 'class == String'
    or '__isString' will not work with ImmutableStrings.
    A somewhat better approach would be to either add a flag to the object (mutability)
    and check this dynamically (expensive) or to place immutable objects into a readonly
    memory segment (the good solution). We will eventually implement the second in the future...

Related information:

    ImmutableArray
    Parser
    Scanner

Instance protocol:

 at: index put: value

Trigger an error if an immutable string is stored into.
The store will be performed (for compatibility reasons) if you continue
in the debugger.

 basicAt: index put: value

Trigger an error if an immutable string is stored into.
The store will be performed (for compatibility reasons) if you continue
in the debugger.

 asImmutableString

 copyEmpty

when copying, return a real (mutable) String

 copyEmptyAndGrow: size

when copying, return a real (mutable) String

 postCopy

when copied, make me a real (mutable) String

 postDeepCopy

when copied, make it me a real (mutable) String

 shallowCopy

when copying, return a real (mutable) String

 creator

find the method that contains me.
This works only, if the string is included in a method's literal array.
Stc compiled code does not put strings into the literal array - Bytecode compiled code does.

 noModificationError

a store is attempted - for our convenience, find the method that
contains me, for a nicer error message

 species

Copies should be mutable

 become: anotherObject

trigger an error if I should become something else
(this would be an even more tricky manipulation)

 becomeNil

trigger an error if I should become nil
(this would be an even more tricky manipulation)