C++ most efficient way of transforming string to hex string

Question

I have this:

std::string TCPMessengerServer::hexStr(unsigned char *data, int len)
{

    constexpr char hexmap[] = { '0', '1', '2', '3', '4', '5', '6', '7',
        '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };

    std::string s(len * 2, ' ');
    for (int i = 0; i < len; ++i) {
        s[2 * i] = hexmap[(data[i] & 0xF0) >> 4];
        s[2 * i + 1] = hexmap[data[i] & 0x0F];
    }
    return s;
}

However, I would like to know if there is a more efficient way of doing this. I'd also like to know if there are easier ways, but if so, what performance trade-offs they present.

Avoiding the possible dynamic memory allocation will be more efficient for strings over a few chars long. — Cheers and hth. - Alf
– Cheers and hth. - Alf, Commented Jan 24, 2016 at 20:16
Why not use a std::ostringstream and the std::hex stream manipulator? — Some programmer dude
– Some programmer dude, Commented Jan 24, 2016 at 20:18
@Cheersandhth.-Alf Maybe true, but unless this function is called hundred of times each second I'd say go for simplicity. — Some programmer dude
– Some programmer dude, Commented Jan 24, 2016 at 20:23

Cheers and hth. - Alf · Accepted Answer · 2016-01-24 20:26:31Z

0

Avoiding the possible dynamic memory allocation will be more efficient for strings over a few chars long.

A good way to avoid when possible, is to let the caller pass a buffer to place the result in, and to provide a default wrapper using dynamic allocation of a suitable buffer (e.g. a std::string, as in your code).

However, unless measurements show this function to be absolutely critical performance-wise, that's most probably just wasted work: it's not something to do as a matter of course.

answered Jan 24, 2016 at 20:26

Cheers and hth. - Alf

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