CODEWITHSUNDEEP
c++
Leif Neland
Leif Neland

Reputation: 1508

std::hex does not work as I expect

I'm not used to C++, so bear with me...

Two bytes are read from a device and gets in a buffer. It is then to be printed.

The code below is supposed to return the string "0x204D" However, it returns "0x M" which in hex is 30 78 20 4d

So the hex is not decoded to ascii.

void vito_unit::decodeAsRaw(unsigned char *buffer, int bufferLen)
{
    std::stringstream *decodedClearText;
    decodedClearText = new std::stringstream;

    *decodedClearText << "0x" << std::hex;

    for (int i=0; i<bufferLen; i++) {
            *decodedClearText << buffer[i];
    }
    setValue(decodedClearText->str());
}

How should it be done?

Upvotes: 3

Views: 5817

Answers (3)

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385144

This has nothing to do with std::hex.

When you stream a [signed/unsigned] char, its ASCII representation is used, because that is usually what is expected of chars.

You can stream a number instead by converting it to int. Then the feature that renders numbers in hexadecimal notation (i.e. std::hex) will be triggered.

You should also fix that memory leak and unnecessary dynamic allocation:

void vito_unit::decodeAsRaw(unsigned char const* const buffer, int const bufferLen)
{
    std::stringstream decodedClearText;
    decodedClearText << "0x" << std::hex;

    for (int i = 0; i < bufferLen; i++) {
       decodedClearText << +buffer[i];
    }

    setValue(decodedClearText.str());
}

The unary "+" performs an integral promotion to int.

Upvotes: 5

Leif Neland
Leif Neland

Reputation: 1508

The hint from Bo Persson was what I needed.

 for (int i=0; i<bufferLen; i++) {
        *decodedClearText << (int)buffer[i];
}

did the trick.

Upvotes: 3

Raeynd
Raeynd

Reputation: 126

buffer[i] is of type unsigned char and is thus printed as a character instead of its hexadecimal representation. You can cast the value to an unsigned int to avoid that.

void vito_unit::decodeAsRaw(unsigned char *buffer, int bufferLen)
{
    std::stringstream *decodedClearText;
    decodedClearText = new std::stringstream;

    *decodedClearText << "0x" << std::hex;

    for (int i=0; i<bufferLen; i++) {
            *decodedClearText << (unsigned int) buffer[i];
    }
    setValue(decodedClearText->str());
}

Upvotes: 4

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