Why Does Go Throw a \'runtime error: makeslice: cap out of range\' Error When Creating a Slice with a Length Greater Than Its Capacity?

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

When defining a slice in Go, the capacity represents the maximum number of elements it can hold. This capacity cannot be less than the length of the slice, as the slice is essentially a reference to a portion of an underlying array.

If we attempt to create a slice with a length greater than its specified capacity, a runtime error will occur with the message "runtime error: makeslice: cap out of range". This is because there is no underlying array with sufficient size to support the requested slice.

The invariant for a slice s in Go is:

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

This means that the length of the slice must always be less than or equal to its capacity.

The reason why this error occurs at runtime instead of compile time is due to potential dynamic allocation. In some cases, the values passed to the make function to create the slice may not be known until runtime. This can happen when using functions like rand.Int() to generate random lengths and capacities.

Therefore, to avoid runtime errors, it is essential to ensure that the capacity of a slice is always greater than or equal to its length.

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