How Efficient is []byte('something') Conversion in Go?

[]byte Conversion Performance

The purpose of the []byte("something") type conversion is to convert a string to a slice of bytes. This conversion is not a method call or function invocation but rather a direct type conversion.

Contrary to what one might expect, the conversion itself does not copy the value of the string. However, converting a string to a []byte still incurs a performance cost due to the need for copying. This is because the resulting byte slice is mutable, and if a copy were not made, any modifications to the byte slice could potentially alter the contents of the immutable string, violating the principles outlined in the Go specification.

While the byte slice conversion generally requires a copying operation, there are specific instances where the compiler optimizes this copy away. These optimizations occur when it can be proven that the immutable string cannot or will not be modified through the conversion.

One example of such an optimization is when looking up a key from a map with a string key type using a []byte, converted to a string, as the index (as shown in the code snippet provided in the given answer). Another optimization is when iterating over the bytes of a string explicitly converted to a byte slice using a for range loop. These optimizations allow for improved performance in specific use cases.

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