Memory Management

• Memory stores programs; i.e., instructions and data
• The Operating System and the C++ runtime manages memory

Quiz: What is a variable?

A pair of a name with its semantics and a corresponding location for storage, normally memory

Quiz: What is a pointer?

A variable that stores a memory address

Quiz: Assuming a 64-bit address space, what are the sizes of all variables in the figure?

Parts of a Program in Memory

• According to ELF, programs are divided into segments (memory), sections (linker)

Text and Operating System parts

• Operating System: The operating system restricts some memory area for itself
• Text: Memory segment storing the code of our program; e.g., main function

Data, Heap, and Stack and variable scopes

• Data: Area storing global variables with lifetime equal to the program
• Heap: Area storing variables dynamically allocated, unknown lifetime and not function limited
• Stack: Area storing variables created within the scope of a function

Example of variables and their section mapping

Heap and Stack example

Quiz: Pointers' storage

Can the heap, stack and data sections store pointers?

Quiz: What is the output of this code?

C++ parameters default pass

In C++, pass-by-copy is the default policy

Quiz: What is the output of this code?

Pointers to the rescue

Pointers can be dangerous

nullptr, the pointer literal

• nullptr represents the null pointer constant
• Its type is std::nullptr_t can be converted to any pointer type
• Never use NULL or 0 for pointers

NULL bad use example

References

• An alias to an already-existing object or function
• Access to references are the same to the object they point to

Reference example

Pointers and Arrays

Arrays

• Definition: contiguously allocated nonempty sequence of objects with a particular element type. Size cannot change
• Example: int array[3]={0, 1, 2};
• Normally, they will be allocated in the stack
• Please prefer std::array container over arrays

Dynamic Memory Allocation

• Dynamic memory is stored in the heap section. Enlarges and reduces at run time
• Allocation:

  int * ptr_int = new int;

  int * array_int = new int[4];

• Deallocation:

  delete ptr_int;

  delete [] array_int;

Pointer Arithmetic

• C++ allows arithmetic operations on pointers
• a[i] == *(a + i * sizeof(a[0]))

Quiz: what is the final value of these pointers

Please assume all pointers point to address 16

uint8_t *ptr = 16; ptr+=2;

int *ptr = 16; ptr=nullptr;

int *ptr = 16; ptr+=3;