why i am getting output blank? - c++

Question

why I am getting output blank? pointers are able to modify but can't read.why?
#include <iostream>
using namespace std;
int main(){
int a = 0;
char *x1,*x2,*x3,*x4;
x1 = (char *)&a;
x2 = x1;x2++;
x3 = x2;x3++;
x4 = x3;x4++;
*x1=1;
*x2=1;
*x3=1;
*x4=1;
cout <<"#" << *x1 << " " << *x2 << " " << *x3 << " " << *x4 << "#"<<endl ;
cout << a << endl;
}
[Desktop]👉 g++ test_pointer.cpp
[Desktop]👉 ./a.out
# #
16843009
I want to read the value of integer by using pointers type of char.
so i can read byte by byte.

Answer 1

You're streaming chars. These get automatically ASCII-ised for you by IOStreams*, so you're seeing (or rather, not seeing) unprintable characters (in fact, all 0x01 bytes).
You can cast to int to see the numerical value, and perhaps add std::hex for a conventional view.
Example:
#include <iostream>
#include <iomanip>
int main()
{
int a = 0;
// Alias the first four bytes of `a` using `char*`
char* x1 = (char*)&a;
char* x2 = x1 + 1;
char* x3 = x1 + 2;
char* x4 = x1 + 3;
*x1 = 1;
*x2 = 1;
*x3 = 1;
*x4 = 1;
std::cout << std::hex << std::setfill('0');
std::cout << '#' << std::setw(2) << "0x" << (int)*x1
<< ' ' << std::setw(2) << "0x" << (int)*x2
<< ' ' << std::setw(2) << "0x" << (int)*x3
<< ' ' << std::setw(2) << "0x" << (int)*x4
<< '#' << '\n';
std::cout << "0x" << a << '\n';
}
// Output:
// #0x01 0x01 0x01 0x01#
// 0x1010101
(live demo)
Those saying that your program has undefined are incorrect (assuming your int has at least four bytes in it); aliasing objects via char* is specifically permitted.
The 16843009 output is correct; that's equal to 0x01010101 which you'd again see if you put your stream into hex mode.
N.B. Some people will recommend reinterpret_cast<char*>(&a) and static_cast<int>(*x1), instead of C-style casts, though personally I find them ugly and unnecessary in this particular case. For the output you can at least write +*x1 to get a "free" promotion to int (via the unary + operator), but that's not terribly self-documenting.
* Technically it's something like the opposite; IOStreams usually automatically converts your numbers and booleans and things into the right ASCII characters to appear correct on screen. For char it skips that step, assuming that you're already providing the ASCII value you want.

Answer 2

Assuming an int is at least 4 bytes long on your system, the program manipulates the 4 bytes of int a.
The result 16843009 is the decimal value of 0x01010101, so this is as you might expect.
You don't see anything in the first line of output because you write 4 characters of a binary value 1 (or 0x01) which are invisible characters (ASCII SOH).
When you modify your program like this
*x1='1';
*x2='3';
*x3='5';
*x4='7';
you will see output with the expected characters
#1 3 5 7#
926233393
The value 926233393 is the decimal representation of 0x37353331 where 0x37 is the ASCII value of the character '7' etc.
(These results are valid for a little-endian architecture.)

Answer 3

You can use unary + for converting character type (printed as symbol) into integer type (printed as number):
cout <<"#" << +*x1 << " " << +*x2 << " " << +*x3 << " " << +*x4 << "#"<<endl ;
See integral promotion:

Answer 4

Have a look at your declarations of the x's
char *x1,*x2,*x3,*x4;
these are pointers to chars (characters).
In your stream output they are interpreted as printable characters.
A short look into the ascii-Table let you see that the lower numbers are not printeable.
Since your int a is zero also the x's that point to the individual bytes are zero.
One possibility to get readeable output would be to cast the characters to int, so that the stream would print the numerical representation instead the ascii character:
cout <<"#" << int(*x1) << " " << int(*x2) << " " << int(*x3) << " " << int(*x4) << "#"<<endl ;

Answer 5

If I understood your problem correctly, this is the solution
#include <stdio.h>
#include <iostream>
using namespace std;
int main(){
int a = 0;
char *x1,*x2,*x3,*x4;
x1 = (char*)&a;
x2 = x1;x2++;
x3 = x2;x3++;
x4 = x3;x4++;
*x1=1;
*x2=1;
*x3=1;
*x4=1;
cout <<"#" << (int)*x1 << " " << (int)*x2 << " " << (int)*x3 << " " << (int)*x4 << "#"<<endl ;
cout << a << endl;
}