What are all the common undefined behaviours that a C++ programmer should know about?

In C++, there are some undefined behaviors. These are identified by doing some tasks in C++. There are no such direct definitions. These few things should be known to all of the programmers, who want to use C++ for different purposes.

Here we will see some C++ Codes. and try to guess the results. The codes will generate some runtime errors.

The Divide By Zero error is undefined.

Example Code

#include <iostream>
using namespace std;
int main() {
   int x = 10, y = 0;
   int z = x / y;
   cout << "Done" << endl;
}

Output

Runtime error for divide by zero operation

Trying to use uninitialized variable.

Example Code

#include <iostream>
using namespace std;
int main() {
   bool x;
   if(x == true)
      cout << "true value";
   else
      cout << "false value";
}

Output

false value (This may differ in different compilers)

Trying to access null pointer values.

Example Code

#include <iostream>
using namespace std;
int main() {
   int *ptr = NULL;
   cout << "The pointer value is: " << *ptr;
}

Output

Runtime error for accessing null pointer values

Trying to access null pointer values.

Example Code

#include <iostream>
using namespace std;
int main() {
   int array[10];
   for(int i = 0; i<=10; i++) {
      cout << array[i] << endl;
   }
}

Output

Runtime error for accessing item out of bound.
Some compiler may return some arbitrary value, not return any error
Going beyond limit of signed int.

Example Code

#include <iostream>
using namespace std;
int main() {
   int x = INT_MAX;
   cout << "x + 1: " << x + 1;
}

Output

x + 1: -2147483648
circulate to the minimum number of signed int

Trying to change some character in String literals.

Example Code

#include <iostream>
using namespace std;
int main() {
   char *str = "Hello World";
   str[2] = 'x';
   cout << str;
}

Output

Runtime error because we are trying to change the value of some constant variables.