How to understand the following c++ code ?

Question

inpfile>>ch;

if(ch<16) outfile<<"0×0"<<std::hex<<setprecision(2)<<(int)ch<<" ";

what does std::hex<<setprecision(2) mean?

Answer 1

std::hex sets the output base to hexadecimal.

setprecision has no effect on this line since it affects floating point only.

Answer 2

iostreams can be manipulated to achieve the desired formatting - this is done by what at first sight looks like outputting predefined values to them as shown in our subject line of code.

std::hex displays following integer values in base16.

setprecision sets the precision for display of following floating values.

Fur further info on manipulators, start here

Answer 3

what does std::hex<<setprecision(2) mean?

std::hex and std::setprecision are both so-called manipulators. Applied to a stream (done by outputting them) they manipulate the stream, usually to change the stream's formatting. In particular, std::hex manipulates the stream so that values are written in hexadecimal, and std::setprecision(x) manipulates it to output numbers with x digits.
(A rather popular manipulator which you might already know about is std::endl.)

As you can see, there are manipulators that take arguments and those that take none. Also, most (in principle all) manipulators are sticky, which means their manipulation of the stream lasts until it is explicitly changed. Here is an extensive discussion about this topic.

Answer 4

This line is the same as:

char  ch;
inpfile>>ch;

if(ch<16)
{
    outfile     << "0×0"             // Prints "0x0" (Ox is the standard prefix for hex numbers)
    /*outfile*/ << std::hex          // Tells the stream to print the next number in hex format
    /*outfile*/ << setprecision(2)   // Does nothing. Presumably they were trying to indicate print min of 2 characters
    /*outfile*/ << (int)ch           // Covert you char to an integer (so the stream will print it in hex
    /*outfile*/ << " ";              // Add a space for good measure.
}

Rather than setprecision(2) what was probably intended was setw(2) << setfill('0')