How to Efficiently Cast a CGO Array of Doubles to a Go Slice of float64?

Casting a CGO Array to a Slice in Go

When working with data structures that originate from C code in Go, you may encounter the need to convert a C-style array into a Go slice. In this context, the underlying question is: can we find a more efficient way to cast a CGO array of doubles into a slice of float64 than the clumsy method below:

doubleSlc := [6]C.double{}

// Fill doubleSlc

floatSlc := []float64{float64(doubleSlc[0]), float64(doubleSlc[1]), float64(doubleSlc[2]),
                      float64(doubleSlc[3]), float64(doubleSlc[4]), float64(doubleSlc[5])}

The answer lies in exploring alternative casting techniques:

The Safe and Portable Method

For a safe and portable solution, one can opt for the following approach:

c := [6]C.double{ 1, 2, 3, 4, 5, 6 }
fs := make([]float64, len(c))
for i := range c {
        fs[i] = float64(c[i])
}

This technique iterates over the CGO array and manually assigns each element to the slice, ensuring type safety.

The Unportable Shortcut

Alternatively, for a less conventional and potentially risky solution, there is:

c := [6]C.double{ 1, 2, 3, 4, 5, 6 }
cfa := (*[6]float64)(unsafe.Pointer(&c))
cfs := cfa[:]

Here, we exploit the fact that both C.double and float64 occupy the same underlying memory layout (if this is indeed the case in your specific scenario). Using unsafe pointers, we cast the CGO array to the corresponding float64 array and slice it.

Caution: This unsafe approach should be used with care as it can lead to undefined behavior if the assumptions about memory layout are incorrect.

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