实现函数如下：

std::string CStringToUTF8(const CString& str)
{
	// Step 1: 将 MBCS (GBK) CString 转换为宽字符（WCHAR）
	int wideLen = MultiByteToWideChar(CP_ACP, 0, str.GetString(), -1, NULL, 0);
	if (wideLen <= 0)
		return "";

	std::wstring wideStr(wideLen, L'\0');
	MultiByteToWideChar(CP_ACP, 0, str.GetString(), -1, &wideStr[0], wideLen);

	// Step 2: 将宽字符字符串转换为 UTF-8 编码的 std::string
	int utf8Len = WideCharToMultiByte(CP_UTF8, 0, wideStr.c_str(), -1, NULL, 0, NULL, NULL);
	if (utf8Len <= 0)
		return "";

	std::string utf8Str(utf8Len, '\0');
	WideCharToMultiByte(CP_UTF8, 0, wideStr.c_str(), -1, &utf8Str[0], utf8Len, NULL, NULL);

	return utf8Str;
}