01_CppCourse.md

Undefined Behavior in C and C++

1. Division by zero

    int val = 5;
    return val / 0;			// undefined behavior

2. Memory accesses outside of array bounds

    int arr[4] = { 1,2,3,4 };
    return arr[5];			// undefined behavior

3. Signed integer overflow

    int x = INT_MAX;		// INT_MAX is the maximum possible integer
    printf("%d", x + 1);	// undefined behavior

4. Null pointer dereference

    val = 0;
    int ptr = *val;		// undefined behavior

5. Modification of a string literal

    char* s = "eni";		// stored in read-only section of memory
    s[0] = 'e';         // undefined behavior

6. Accessing a NULL pointer

    int* ptr;
    printf("%d", *ptr);		// undefined behavior

7. Misuse of the delete operator with an array allocated using new[]

When you allocate an array with new[], you must deallocate it with delete[]. Using delete on an array can lead to partial deallocation of the array, potentially causing a memory leak.

    auto p = new int[n];	  // dynamically allocates memory for an array of 'n' integers
    delete p;				     // undefined behavior

8. Undefined behavior in bitwise shifting

• Shifting more bits than size of type.

   unsigned int a = 1;
   unsigned int result = a << 32;

• Shifting a 32-bit integer by 32 or more bits is undefined behavior.

    int a = 1;
    int shift_amount = -1;
    int result = a << shift_amount;

• Shifting by a Negative amount is undefined behavior.

    int a = 1;
    int shift_amount = -1;
    int result = a << shift_amount;

• Left-shifting a negative value is undefined behavior.

    int a = -1;
    int result = a << 1;

• Shifting into the sign bit is undefined behavior.

    int a = 1 << 30;
    int result = a << 1;