The infinite Loop Problem

[SapphireSpire]

I'm just testing some ideas for accessing the bytes and bits of an integer. Everything works, the only problem is that I get an infinite loop if I enter a value greater than what an int can hold, like 4,300,000,000. The 'if' statement on line 24 doesn't seem to catch it.

#include <iostream>
#include <bitset>
using namespace std;

#define num unsigned int
#define let unsigned char

int main() {
    while (true) { //begin loop
        num x = 0;
        cout << "Press 1 to break or 2 to continue ";
        cin >> x;
        if (x == 1) break;

        //create an integer and a char array in a union
        union {
            num y = 0;
            let a[4];
        };

        //get a value for y
        cout << endl << endl << "Enter a value between 0 and 4,294,967,295" << endl;
        cin >> y;
        if(cin.fail()) cin.clear();

        //output the bytes of y as unsigned integers big-endian
        cout << (num)a[3] << " ";
        cout << (num)a[2] << " ";
        cout << (num)a[1] << " ";
        cout << (num)a[0] << " ";

        cout << endl << endl;

        //create a pointer to y
        let* p = (let*)&y;

        //output the bits of each byte of y big-endian
        bitset<8> four(*(p + 3));
        cout << four << " ";        //4th byte of the integer
        bitset<8> three(*(p + 2));
        cout << three << " ";       //3rd byte of the integer
        bitset<8> two(*(p + 1));
        cout << two << " ";     //2nd byte of the integer
        bitset<8> one(*p);
        cout << one << " ";     //1st byte of the integer

        cout << endl << endl;

        //using a for loop to output the bytes of y as integers big-endian
        for (int i = 3; i >= 0; i--) {
            cout << (num)*(p + i) << " ";
        } //end for

        cout << endl << endl;

    } //end loop
    return 0;
} //end main 

You should be able to copy and paste this text into an empty .cpp file in Visual Studio 2015 Community Edition and run it (F5).

[Chervil]

Where is num defined please?

[SapphireSpire]

Opps! All of my preprocessor directives got lost.

There you go! :D

[TheIdeasMan]

Compilation with cpp.sh (The gear icon top left of the code) with all 3 warnings on:

 In function 'int main()': 
24:14: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits] 
24:23: warning: comparison is always false due to limited range of data type [-Wtype-limits]

Let the compiler work for you.

#define num unsigned int
#define let unsigned char 

Don't do that, it just provides obfuscation.

What is the point if checking for < 0 , when the type is unsigned ?

[SapphireSpire]

In case someone enters a negative value - lol. hmmm

[Chervil]

If you need to test for negative values, you need to use a datatype capable of holding negative values.

All values will be in the range 0 to 4294967295 (for a 32-bit unsigned integer), so testing for anything outside that range won't work.

The reason for the infinite loop is that if the entered value is too large, the cin stream will have the fail() flag set. Before continuing with another cin operation, you'd need to clear the flag. Or quit from the loop, if that is your preference.

        cin >> y;
        if (!cin)
        {
            cin.clear(); // reset flags after invalid input
        }

You might also want to trap non-numeric input, which could also cause problems.

        num x = 0;
        
        cout << "Press 1 to break or 2 to continue ";
        
        while (!(cin >> x))
        {
            cin.clear();
            cin.ignore(1000, '\n');            
            cout << "Press 1 to break or 2 to continue ";
        }

[SamuelAdams]

Line 24 will never work since it's out of bounds.

You have to check the value before you assign it.