CODEWITHSUNDEEP
c++arraysintegercharunsigned
Blue42
Blue42

Reputation: 355

C++ copying integer to char[] or unsigned char[] error

So I'm using the following code to put an integer into a char[] or an unsigned char[]

(unsigned???) char test[12];

test[0] = (i >> 24) & 0xFF;
test[1] = (i >> 16) & 0xFF;
test[2] = (i >> 8) & 0xFF;
test[3] = (i >> 0) & 0xFF;

int j = test[3] + (test[2] << 8) + (test[1] << 16) + (test[0] << 24);

printf("Its value is...... %d", j);

When I use type unsigned char and value 1000000000 it prints correctly.

When I use type char (same value) I get 98315724 printed?

So, the question really is can anyone explain what the hell is going on??

Upon examining the binary for the two different numbers I still can't work out whats going on. I thought signed was when the MSB was set to 1 to indicate a negative value (but negative char? wth?)

I'm explicitly telling the buffer what to insert into it, and how to interpret the contents, so don't see why this could be happening.

I have included binary/hex below for clarity in what I examined.

11 1010 1001 1001 1100 1010 0000 0000 // Binary for 983157248

11 1011 1001 1010 1100 1010 0000 0000 // Binary for 1000000000

3 A 9 9 C A 0 0 // Hex for 983157248

3 B 9 A C A 0 0 // Hex for 1000000000

Upvotes: 0

Views: 703

Answers (3)

soldari
soldari

Reputation: 1

This all has to do with internal representation and the way each type uses that data to interpret it. In the internal representation of a signed character, the first bit of your byte holds the sign, the others, the value. when the first bit is 1, the number is negative, the following bits then represent the complement of the positive value. for example:

unsigned char c;  // whose internal representation we will set at 1100 1011
c = (1 * 2^8) + (1 * 2^7) + (1 * 2^4) + (1 * 2^2) + (1 * 2^1);
cout << c;        // will give 203

                  // inversely:

char d = c;       // not unsigned
cout << d;        // will print -53
                  // as if the first is 1, d is negative, 
                  // and other bits complement of value its positive value
                  // 1100 1011  -> -(complement of 100 1011)
                  // the complement is an XOR +1   011 0101

                  // furthermore:

char e;           // whose internal representation we will set at 011 0101
e = (1 * 2^6) + (1 * 2^5) + (1 * 3^2) + (1 * 2^1);
cout << e;        // will print 53

Upvotes: 0

Nik Bougalis
Nik Bougalis

Reputation: 10613

In addition to the answer by Kerrek SB please consider the following:

Computers (almost always) use something called twos-complement notation for negative numbers, with the high bit functioning as a 'negative' indicator. Ask yourself what happens when you perform shifts on a signed type considering that the computer will handle the signed bit specially.

You may want to read Why does left shift operation invoke Undefined Behaviour when the left side operand has negative value? right here on StackOverflow for a hint.

Upvotes: 0

Kerrek SB
Kerrek SB

Reputation: 477090

When you say i & 0xFF etc, you're creaing values in the range [0, 256). But (your) char has a range of [-128, +128), and so you cannot actually store those values sensibly (i.e. the behaviour is implementation defined and tedious to reason about).

Use unsigned char for unsigned values. The clue is in the name.

Upvotes: 0

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