Making UnsafeMutablePointer<UInt8> from String without Foundation.

string2cstring.swift

// String → UTF8View → UnsafeMutablePointer<UInt8>
// Note: UTF8View.Element == UInt8
// Note: utf8CString: ContigiousArray<Int8>

func makeCString(from str: String) -> UnsafeMutablePointer<UInt8> {
    var utf8 = Array(str.utf8)
    utf8.append(0)  // adds null character
    let count = utf8.count
    let result = UnsafeMutableBufferPointer<UInt8>.allocate(capacity: count)
    _ = result.initialize(from: utf8)
    return result.baseAddress!
}


do {
    let str = "abc abc"
    let cstr = makeCString(from: str)
    let restr = String(cString: cstr)
    print(str == restr) // true

    let str2 = "世界の皆様 おはようございます。 🌄 Good morning!"
    let cstr2 = makeCString(from: str2)
    let restr2 = String(cString: cstr2)
    print(str2 == restr2) // true

    let str3 = "✑ 🍍 🍎 ✒"
    let cstr3 = makeCString(from: str3)
    let restr3 = String(cString: cstr3)
    print(str3 == restr3) // true
}

string2cstring2.swift

// Method 2
// String → UnsafePointer<UInt8> → UnsafeMutablePointer<UInt8>
// func withCString<Result, TargetEncoding>(encodedAs targetEncoding: TargetEncoding.Type, _ body: (UnsafePointer<TargetEncoding.CodeUnit>) throws -> Result) rethrows -> Result where TargetEncoding : _UnicodeEncoding
// Note: UTF8.CodeUnit = UInt8

func makeCString2(from str: String) -> UnsafeMutablePointer<UInt8> {
    let count = str.utf8.count + 1
    let result = UnsafeMutablePointer<UInt8>.allocate(capacity: count)
    str.withCString(encodedAs: UTF8.self) { (baseAddress) in 
        // baseAddress: UnsafePointer<UInt8>
        result.initialize(from: baseAddress, count: count)
    }
    return result
}

do {
    let str = "abc abc"
    let cstr = makeCString2(from: str)
    let restr = String(cString: cstr)
    print(str == restr) // true

    let str2 = "世界の皆様 おはようございます。 🌄 Good morning!"
    let cstr2 = makeCString2(from: str2)
    let restr2 = String(cString: cstr2)
    print(str2 == restr2) // true

    let str3 = "✑ 🍍 🍎 ✒"
    let cstr3 = makeCString2(from: str3)
    let restr3 = String(cString: cstr3)
    print(str3 == restr3) // true
}