A function to convert a CFString (macOS / OS X) to a UTF-8 std::string in C++.

cfStringToStdString.cpp

#include <CoreFoundation/CoreFoundation.h>
#include <string>
#include <vector>

/**
 * Converts a CFString to a UTF-8 std::string if possible.
 *
 * @param input A reference to the CFString to convert.
 * @return Returns a std::string containing the contents of CFString converted to UTF-8. Returns
 *  an empty string if the input reference is null or conversion is not possible.
 */
std::string cfStringToStdString(CFStringRef input)
{
    if (!input)
        return {};

    // Attempt to access the underlying buffer directly. This only works if no conversion or
    //  internal allocation is required.
    auto originalBuffer{ CFStringGetCStringPtr(input, kCFStringEncodingUTF8) };
    if (originalBuffer)
        return originalBuffer;

    // Copy the data out to a local buffer.
    auto lengthInUtf16{ CFStringGetLength(input) };
    auto maxLengthInUtf8{ CFStringGetMaximumSizeForEncoding(lengthInUtf16,
        kCFStringEncodingUTF8) + 1 }; // <-- leave room for null terminator
    std::vector<char> localBuffer(maxLengthInUtf8);

    if (CFStringGetCString(input, localBuffer.data(), maxLengthInUtf8, maxLengthInUtf8))
        return localBuffer.data();

    return {};
}

It turns out CFStringGetMaximumSizeForEncoding for kCFStringEncodingUTF8 only multiplies by 3, so this fails if the string is entirely 4 byte unicode characters like 💩.
It turns out all 4-byte UTF-8 characters are at least 4-byte/2-code-pair Unicode characters so this does work.

if (CFStringGetCString(input, localBuffer.data(), maxLengthInUtf8, kCFStringEncodingUTF8))
and not
if (CFStringGetCString(input, localBuffer.data(), maxLengthInUtf8, maxLengthInUtf8))