Wrong output when converting string to IP address

Question

I'm trying to convert a string to IP address. The input string is an unsigned integer converted to std::string, for example "123456". The code below is not correct, as it produces unreadable binary characters.

std::string str2IP(const std::string& address)
{
    uint32_t ip = std::strtoul(address.c_str(), NULL, 0);
    unsigned char bytes[4];
    bytes[0] = ip & 0xFF;
    bytes[1] = (ip >> 8) & 0xFF;
    bytes[2] = (ip >> 16) & 0xFF;
    bytes[3] = (ip >> 24) & 0xFF;

    std::stringstream ss;
    ss << bytes[3] << "." << bytes[2] << "." << bytes[1] << "." << bytes[0];
    return ss.str();
}

What output did you get for what input? What output did you expect for that input? (Also, why are you outputting characters?) — David Schwartz
– David Schwartz, Commented Jul 9, 2015 at 22:11

Angew is no longer proud of SO · Accepted Answer · 2015-07-09 22:17:32Z

3

The formatted output functions (operator <<) of I/O streams treat char, signed char, and unsigned char as characters—they interpret the value as a character code, not as a number. This code will output A:

unsigned char c = 65;
std::cout << c;

The same holds for std::uint8_t on most implementations, because they just use it as a typedef to an unsigned char. You need to use a proper numerical type, such as unsigned short:

std::string str2IP(const std::string& address)
{
    uint32_t ip = std::strtoul(address.c_str(), NULL, 0);
    unsigned short bytes[4];
    bytes[0] = ip & 0xFF;
    bytes[1] = (ip >> 8) & 0xFF;
    bytes[2] = (ip >> 16) & 0xFF;
    bytes[3] = (ip >> 24) & 0xFF;

    std::stringstream ss;
    ss << bytes[3] << "." << bytes[2] << "." << bytes[1] << "." << bytes[0];
    return ss.str();
}

answered Jul 9, 2015 at 22:17

Angew is no longer proud of SO

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Comments

TartanLlama · Accepted Answer · 2015-07-09 22:16:35Z

1

Outputting chars to a std::stringstream has the semantics of outputting the encoded character represented by that char rather than a numerical representation.

You can force numerical representation by using unary plus to promote those chars:

ss << +bytes[3] << "." << +bytes[2] << "." << +bytes[1] << "." << +bytes[0];

answered Jul 9, 2015 at 22:16

TartanLlama

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1 Comment

Loki Astari Over a year ago

That will work. But that's not a good idea for nice readable portable/maintainable code. Change the type of bytes or explicitly cast them (static_cast) to make it obvious what your are doing.

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