C++ Beginner : I can't fix&compile the code - c++

Question

I have been started C++ lessons in my University since 3 weeks, so I'm quite beginner.
I have 4 codes to find the errors and fix them, 2 of them are done, but i couldn't figure out why did not the other 2 get compiled.
There's the first code;
#include <cstring>
#include <iostream>
#include <climits>
#include <cfloat>
#include <cmath>
using std::cout;
using std::endl;
template<class T>
void to_bin(T v)
{
union
{
T value;
unsigned char bytes[sizeof(T)];
};
memset(&bytes, 0, sizeof(T));
value=v;
// assumes little endian machine
for (size_t i = sizeof(T);i>0;i--)
{
unsigned char pot=128;
for (int j = 7;j>=0;j--,pot/=2)
if (bytes[i-1]&pot)
cout << "1";
else
cout << "0";
cout << " ";
}
cout << endl;
}
int main() {
cout <<" Binaerdarstellungen von -2, -1, 0, 42, INT_MAX und INT_MAX+1 als Int: " << endl;
int p_i = (-2), q_i=-1,r_i=0,s_i=42,t_i=INT_MAX,u_i=INT_MAX+1 ;
cout << "Fuer -2: ";
to_bin(p_i);
cout << endl;
cout << "Fuer -1: ";
to_bin(q_i);
cout << endl;
cout << "Fuer 0: ";
to_bin(r_i);
cout << endl;
cout << "Fuer 42: ";
to_bin(s_i);
cout << endl;
cout << "Fuer INT_MAX: ";
to_bin(t_i);
cout << "Fuer INT_MAX+1: ";
to_bin(u_i);
cout << endl;
cout << endl << "Binaerdarstellungen von -2.0, 1.0, 0.0, 42.0, FLT_MAX, FLT_MAX+1 als float: " << endl;
float p_f = -2.0, q_f= -1.0,r_f=0.0,s_f=42.0,t_f=FLT_MAX,u_f=FLT_MAX+1 ;
cout << "Fuer -2.0: ";
to_bin(p_f);
cout << endl;
cout << "Fuer -1.0: ";
to_bin(q_f);
cout << endl;
cout << "Fuer 0.0: ";
to_bin(r_f);
cout << endl;
cout << "Fuer 42.0: ";
to_bin(s_f);
cout << endl;
cout << "Fuer FLT_MAX: ";
to_bin(t_f);
cout << endl;
cout << "Fuer FLT_MAX+1: ";
to_bin(u_f);
cout << endl;
cout << endl << "Binaerdarstellungen von -2.0, 1.0, 0.0, 42.0, DBL_MAX, DBL_MAX+1 als double: " << endl;
double p_d = -2.0, q_d=-1.0,r_d=0.0,s_d=42.0,t_d=DBL_MAX,u_d=DBL_MAX+1 ;
cout << "Fuer -2.0: ";
to_bin(p_d);
cout << endl;
cout << "Fuer -1.0: ";
to_bin(q_d);
cout << endl;
cout << "Fuer 0.0: ";
to_bin(r_d);
cout << endl;
cout << "Fuer 42.0: ";
to_bin(s_d);
cout << endl;
cout << "Fuer DBL_MAX: ";
to_bin(t_d);
cout << endl;
cout << "Fuer DBL_MAX+1: ";
to_bin(u_d);
cout << endl;
cout << endl << "Groesse von Integer Variablen: ";
cout << sizeof(p_i) << endl;
cout << "Groesse von Gleitkomma Variablen mit einfacher Genauigkeit: ";
cout << sizeof(p_f) << endl;
cout << "Groesse von Gleitkomma Variablen mit doppelter Genauigkeit: ";
cout << sizeof(p_d) << endl;
return 0;}
and so the other one;
#include <iostream>
#include <cstring>
using namespace std;
template<class T>
void to_bin(T v)
{
using std::cout;
using std::endl;
union
{
T value;
unsigned char bytes[sizeof(T)];
};
memset(&bytes, 0, sizeof(T));
value=v;
// assumes little endian machine
for (size_t i = sizeof(T);i>0;i--)
{
unsigned char pot=128;
for (int j = 7;j>=0;j--,pot/=2)
if (bytes[i-1]&pot)
cout << "1";
else
cout << "0";
cout << " ";
}
cout << endl;
}
int main() {
long unsigned int stack_field;
long unsigned int *heap_field = new long unsigned int;
short unsigned int *stack_pointer = (short unsigned int*)&stack_field;
short unsigned int *heap_pointer = (short unsigned int*)heap_field;
stack_field = 0;
*heap_field = 0;
*stack_pointer = 1;
*heap_pointer = 1;
stack_pointer = stack_pointer + 1;
heap_pointer = heap_pointer + 1;
to_bin(stack_field);
to_bin(*heap_field);
to_bin(*stack_pointer);
to_bin(*heap_pointer);
*stack_pointer = 1;
*heap_pointer = 1;
to_bin(stack_field);
to_bin(*heap_field);
to_bin(*stack_pointer);
to_bin(*heap_pointer);
// HIER EDITIEREN
cout << "stack_field " << sizeof(stack_field) <<" " << stack_field <<" "<< &stack_field << endl;
cout << "heap_field " << sizeof(heap_field) <<" " << *heap_field <<" "<< &heap_field << endl;
cout << "stack_pointer " << sizeof(stack_pointer) <<" " << *stack_pointer <<" "<< &stack_pointer << endl;
cout << "heap_pointer " << sizeof(heap_pointer) <<" " << *heap_pointer <<" "<< &heap_pointer << endl;
delete heap_field;
return 0;
}
What is the thing, that i'm doing wrong?
I only have 3 more hours to finish and upload it.
Thanks...
UPDATE
Errors of first code :
x#cluster:~/x/cppfiles[534]$ g++ -Wall -Werror -o aufgabe1 aufgabe1.cpp
cc1plus: warnings being treated as errors
aufgabe1.cpp: In function 'int main()':
aufgabe1.cpp:41: error: integer overflow in expression
aufgabe1.cpp: In function 'void to_bin(T) [with T = int]':
aufgabe1.cpp:44: instantiated from here
aufgabe1.cpp:17: error: unused variable 'value'
aufgabe1.cpp:17: error: unused variable 'bytes'
aufgabe1.cpp: In function 'void to_bin(T) [with T = float]':
aufgabe1.cpp:70: instantiated from here
aufgabe1.cpp:17: error: unused variable 'value'
aufgabe1.cpp:17: error: unused variable 'bytes'
aufgabe1.cpp: In function 'void to_bin(T) [with T = double]':
aufgabe1.cpp:97: instantiated from here
aufgabe1.cpp:17: error: unused variable 'value'
aufgabe1.cpp:17: error: unused variable 'bytes'
And errors of the second code:
x#cluster:~/x/cppfiles[534]$ g++ -Wall -Werror -o aufgabe4 aufgabe4.cpp
cc1plus: warnings being treated as errors
aufgabe4.cpp: In function 'void to_bin(T) [with T = long unsigned int]':
aufgabe4.cpp:48: instantiated from here
aufgabe4.cpp:15: error: unused variable 'value'
aufgabe4.cpp:15: error: unused variable 'bytes'
aufgabe4.cpp: In function 'void to_bin(T) [with T = short unsigned int]':
aufgabe4.cpp:50: instantiated from here
aufgabe4.cpp:15: error: unused variable 'value'
aufgabe4.cpp:15: error: unused variable 'bytes'

Answer 1

You probably get those eroors because of the -Wall (make gcc display "many" warnings) and -Werror (treat all warnings as errors).
So your code may compile fine if no special flags are given, but here you force adding warning and failing on them.
Fixing the code is then necessary:
for error: unused variable 'value' this is quite simple, just remove the useless variables.
For the overflow, check the value in order not to overflow (fix the code); my compiler points int u_i=INT_MAX+1 which is obviously overrflowing (you explicitly add 1 to the maximum value)
Note: adding -Wall -Werror is a good idea (IMHO) as it generally points coding errors or things that would improve your code.