Why Does Exceeding the Capacity of a Go Slice Result in a \'makeslice: cap out of range\' Runtime Error?

Why Does Slicing with Length Greater Than Capacity Result in a Runtime Error?

When creating a slice, it is imperative that the capacity, or the underlying array size, is not less than the length, or the number of elements in the slice. Attempting to do so, as seen in the provided Go program, triggers a "makeslice: cap out of range" runtime error.

The reason for this constraint lies in the very nature of slices. Slices are not stand-alone data structures but rather references to a section of an underlying array. By definition, the capacity of a slice is the size of this backing array. If the length of the slice ever exceeds its capacity, there would be no corresponding memory to store the elements. This logical inconsistency results in the runtime error.

To maintain consistency, the Go compiler enforces the invariant:

0 <= len(s) <= cap(s)

for every slice s. In the provided code, however, the attempt to access the element at index 8 with a capacity of only 5 violates this invariant, triggering the runtime error.

It's important to note that this invariant cannot always be statically verified, as in cases where the capacity and length are determined at runtime. This is precisely why, in certain scenarios, the error manifests as a runtime error instead of a compile-time error.

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