Chapter 08Data Structure InternalsPaid chapter

Strings, Bytes, and the Cost of Conversion

Understand when string ↔ []byte conversions allocate, which proofs elide the copy on modern Go, and how to minimize conversion overhead.

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What this chapter teaches

Common belief

string(b) and []byte(s) are basically free reinterpretations.

What actually happens

they copy by default — but the compiler elides the copy wherever it can prove it unobservable (transient uses, never-written slices, small non-escaping results), and that proof set has grown across releases. Retention and mutation are what always cost.

Mechanisms covered

What always copies: a result that is retained (map store, return, struct field) or bytes that may be mutated before the string dies.

strings.Builder avoids repeated allocation by growing a []byte buffer and converting once via unsafe at the end. unsafe.String/unsafe.SliceData (Go 1.20+) give zero-copy conversion if the immutability invariant is upheld manually.

Conversion benchmarks need >32-byte data or the stack buffer hides the cost; the lab pins every alloc count in TestConversionAllocs.

benchbench + compileexperiment

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Escape Analysis — The Compiler's Fragile Decision

Understand how the compiler decides between stack and heap allocation, why the decision is fragile, and how to verify it.

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Maps — Swiss Tables, Growth, and Permanent Memory

Understand the Go 1.24+ Swiss Table map layout, why maps don't shrink, and how to design for memory reclamation.