Cast int into an array of two bytes

Question

I'm a Java programmer and I tried to use two different way of printing bytes into stdout.

unsigned char bytes[2];
//...
printf("%x%x", bytes[0], bytes[1]);
std::cout << bytes[0] << bytes[1];

But the output of these methods is different. Why? How to make printf output the same as with std::cout ?

Answer 1

With std::cout they will be printed as characters.

With printf integer promotion occurs and they get passed as int s, and the %x specifier tells printf to print an unsigned int in hexadecimal format.

You can get printf to print a character by using %c , or you can get std::cout to print it as hex by doing the promotion yourself and setting the hex flag:

std::printf("%c%c", bytes[0], bytes[1]);
std::cout << std::hex << +bytes[0] << +bytes[1];

Answer 2

%x outputs a number in a hexadecimal CS, for example, code

int x = 80;
printf("%x", x);

will print 50 .

To write numbers in decimal form, you have to use %d . Also, to print char variable in a form of a number, not character, you have to cast it to int :

unsigned char bytes[2];
//...
printf("%d%d", bytes[0], bytes[1]);
std::cout << (int)bytes[0] << (int)bytes[1];

Answer 3

The equivalent would be :

#include <iomanip>  // needed for std::hex

std::cout << std::hex << static_cast<unsigned int>(bytes[0]) << static_cast<unsigned int>(bytes[1]);

Two differences :

• your printf with %x prints in hexadecimal - for C++ streams, there's std::hex for that
• your printf takes int values (for %x ), but C++ streams don't have an operator<< that prints a unsigned char as an integer value (instead it'll be printed as a character), so you need to cast the unsigned char to an integer type first (eg. unsigned int as in the code above)