Notice

The purpose of the following code examples is to demonstrate the semantics and behavior of the C++ language. They may not be complete and are not necessarily structured as they should be in actual code. Please adhere to all coding guidelines in your work.

• Declare methods only in the header (.hpp) file
• Define methods only in the implementation (.cpp) file
• This also applies to constructors and destructors

Object Construction in C++

• Memory for the object is allocated from the stack or the heap
• Memory from the heap is allocated using the operator new
• The constructor initializes the allocated memory
• The constructor does not allocate memory

Memory Allocation and the Constructor

• For the stack:
  1. The program allocates memory from the top of the stack
  2. The constructor is called on the allocated memory in the stack
• For the heap:
  1. The operator new allocates memory from the heap
  2. The constructor is called on the allocated memory from the heap
• The entire memory for a class must be allocated in one contiguous chunk. The memory must be large enough for the memory requirements for all of the fields.
• When compiling a variable declaration, the compiler must know the size of the resulting variable. The sizeof() operator shows the size of a variable or type at compile time.

Object Memory Allocation

• Memory for the entire object is allocated first, including memory for all fields
• For an empty class, the size is always 1 to ensure that the addresses of two different objects will be different
• The size of an object is the sum of the size of the fields plus the padding
• Memory for fields is constructed first
• Must know the fields of a class to know how much memory to allocate

Object Constructor Order

• Constructors for all fields are called before the constructor for the object
• Constructors are called in the order they appear in the class definition

Member Initialization Lists

• Part of the constructor
• Often ignored until after it is needed
• The only way that we can control what constructor is called for the fields of the class
• Syntax is a little strange but necessary

Non-Static Member Initialization

• Introduced in C++11
• Some limits to what types can be initialized and with what expressions
• It does help that you won't forget to initialize
• It does hurt that you cannot have initialization based on constructor parameters
• It does hurt that it exposes implementation details
• Use when appropriate

Good Practice

• Don't wait for a problem to occur
• Use the member initialization list as much as possible
• Use non-static member initialization when appropriate