Memory Management

Memory regions

When a program written with C++ is executed, a section of physical memory is allocated to the program. The program will then partition the memory into regions.

Code Memory Region

The memory region to store code binary.

Static Memory Region

The memory region to store global and static variables.

Stack Memory Region

The memory region to store formal parameters and local variables allocated during function calls. Also known as automatic memory region.

Heap Memory Region

The memory region to store data allocated using the new operator. Also known as dynamic memory region.

Stack and Function Calls

Stack is an ADT in which elements are added and removed in a first in last out order (FILO). It aligned well with the requirement to store data required in function calls. The memory allocated for function calls in C++ is thus named the stack memory region.

In C++, every function call requires the data storage of all formal parameters and local variables. They are packed into a Stack Frame and pushed into the stack when the function is called. The Stack Frame is popped and removed from the stack when the function returns.

Dynamic Memory Allocation

• Data stored in heap

• new creates the data structure in heap memory and returns a pointer

  int *intPtr = new int;  // single variable
  int *array = new int[10];  // array
  MyClass *myObjPtr = new MyClass();  // object
  

• delete destruct the data structure the pointer is pointing to and release the memory in heap

  • always call delete on a pointer (dynamic array is also a pointer)

  • add [] if the pointer is pointing to a list of memory blocks (array)

  delete intPtr;
  delete [] array1;
  delete myObjPtr;
  

• The allocated data in heap are known as dynamic data

Memory Problems

Memory leak

• Only for data stored in the heap region (dynamic data)

• Dynamic data allocated (new) but not released (delete)

• Likely to be overlooked in function and class definitions when the flow of control is not obvious

Other Problems

• Stack overflow because of endless function calls, e.g. recursion

• Out-of-boundary array index

• Oversized C string operations

• Uninitialized pointer access

• Deleting the same pointer multiple times

Memory Problem Checking Tool

Note

Optional reading material. Beneficial to projects depending on working style.

The command line tool valgrind, read instruction here

Dynamic Data and Functions

Refer to the Return an Array section for details on how dynamic data allows returning an array from a function.

You can return dynamic data that are not array also.

Glossary

Dynamic Data

Data allocated in the heap memory region.

Stack Frame

The section of memory allocated in the stack memory region for each function call.

Memory Leak

The memory problem that the memory allocated for dynamic data is not correctly released.