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Force_encoding

As explained in The Well-Grounded Rubyist, Fourth Edition

force_encoding is a Ruby String method that assigns an encoding to a string's existing bytes without validating or converting those bytes.

Ruby’s String#force_encoding assigns a new encoding to a string’s existing bytes without converting or validating those bytes. Unlike encode, it bypasses compatibility checks and applies the requested encoding unconditionally. section 8.1.7

Use force_encoding when byte-level processing produces a string whose encoding metadata needs to be restored:

  • A custom String#^ implementation XORs corresponding bytes, packs them into a string, and calls force_encoding(self.encoding) so the result uses the original string’s encoding. section 10.11.3
  • Do not confuse force_encoding with encode: encode performs permitted transcoding, while force_encoding changes the interpretation assigned to the existing byte sequence. section 8.1.7

Restoring an XOR result’s encoding

class String def ^(key) kenum = key.each_byte.cycle each_byte.map { |byte| byte ^ kenum.next }.pack("C*").force_encoding(self.encoding) end end The byte sequence created by map and pack is therefore labeled with the receiver’s original encoding. section 10.11.3

each_byte.map {|byte| byte ^ kenum.next }.pack("C*").
   force_encoding(self.encoding)

Inspect the complete byte-wise XOR expression, including pack("C*") and force_encoding(self.encoding). section 10.11.3

How The Well-Grounded Rubyist, Fourth Edition covers it

How deep it goes
Explains unconditional assignment and contrasts it with permitted transcoding, then demonstrates the mechanism inside an XOR method.
Examples
Demonstrates force_encoding in a String#^ implementation that maps XORed bytes, packs them, and restores the receiver’s encoding.
What it assumes
Assumes familiarity with Ruby strings, encodings, byte iteration, and basic Enumerable operations.
What you can do after
Apply force_encoding to assign an intended encoding after constructing a string from raw or transformed bytes.
How it connects
Connects force_encoding with String#encode, string encodings, byte iteration, pack, enumerators, and XOR operations.
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of The Well-Grounded Rubyist, Fourth Edition. The section links point to the parts of the book it draws on.