Why does int8_t and user input via cin shows strange result

Question

A tiny piece of code drives me crazy but hopefully you can prevent me from jumping out of the window. Look here:

#include <iostream>
#include <cstdint>

int main()
{
    int8_t i = 65;
    int8_t j;

    std::cout << "i = " << i << std::endl; // the 'A' is ok, same as uchar

    std::cout << "Now type in a value for j (use 65 again): " << std::endl;
    std::cin >> j;
    std::cout << "j = " << j << std::endl;

    if (i != j) 
        std::cout << "What is going on here?????" << std::endl;
    else 
        std::cout << "Everything ok." << std::endl;

    return 0;
}

If I use int instead of int8_t everything ok. I need this as 8-bit unsigned integers, not bigger. And btw. with unsigned char it's the same behaviour - of course - as with int8_t.

Anyone with a hint?

Answer 1

int8_t is a typedef for an integer type with the required characteristics: pure 2's-complement representation, no padding bits, size of exactly 8 bits.

For most (perhaps all) compilers, that means it's going to be a typedef for signed char.(Because of a quirk in the definition of the term signed integer type, it cannot be a typedef for plain char, even if char happens to be signed).

The >> operator treats character types specially. Reading a character reads a single input character, not sequence of characters representing some integer value in decimal. So if the next input character is '0', the value read will be the character value '0', which is probably 48.

Since a typedef creates an alias for an existing type, not a new distinct type, there's no way for the >> operator to know that you want to treat int8_t as an integer type rather than as a character type.

The problem is that in most implementations there is no 8-bit integer type that's not a character type.

The only workaround is to read into an int variable and then convert to int8_t (with range checks if you need them).

Incidentally, int8_t is a signed type; the corresponding unsigned type is uint8_t, which has a range of 0..255.

(One more consideration: if CHAR_BIT > 8, which is permitted by the standard, then neither int8_t nor uint8_t will be defined at all.)