CGO: Convert between LPCWSTR and string

Question

I'm writing CGO bindings for a library which uses LPCWSTR for all of its string types. How do I convert from C.LPCWSTR to string and vice-versa?

Answer 1

You should be able to "cast" the LPCWSTR as a []uint16, and use the utf16 package to decode the characters

// take a C.wstr pointer, and convert it to a go slice
// `sz` is the length of the LPCWSTR
wstr := (*[1 << 30-1]uint16)(unsafe.Pointer(C.wstr))[:sz:sz]
runes := utf16.Decode(wstr)
goString := string(runes)

You generally don't want to be passing Go pointers into your C code, so when converting from a string to an LPCWSTR, you will want to alloc the memory in C. A solution for converting from a Go string s might look like:

func Encode(s string) C.LPCWSTR {
    wstr := utf16.Encode([]rune(s))

    p := C.calloc(C.size_t(len(wstr)+1), C.sizeof_uint16_t)
    pp := (*[1 << 30]uint16)(p)
    copy(pp[:], wstr)

    return (C.LPCWSTR)(p)
}

There may also be some MFC macros that could help convert to and from cstrings which you could take advantage of with simple wrapper functions in C. This way you could easily copy the data in and out just using the builtin C.CString and C.GoString functions.