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4. Structs, Unions, Data Sizes

14/02/23

Structs​

  • structs and classes are virtually the same thing in C++
  • Use struct to create a new structure. and . to access values of it
  • Once defined can use as any other type
  • Can take the address of a variable of type struct and store it in a struct pointer
  • Can embed a struct as a member of another struct
  • Can create an array of structs
  • sizeof() is total of the size of the members

Either pass the struct into a function or a pointer

Allocating memory from the heap​

  • malloc() will allocate bytes of memory
  • Will not (directly) allocate a string for you, or an array, or an int
  • In C++ you have constructors, and may add hidden things to your objects
  • Should store the returned address in a pointer of the type you wish to use it as
  • malloc() returns a void*

Steps to dynamic memory bliss​

  1. Work out how much memory you need to allocate
  2. Ask for that amount of memory
  3. Store the returned pointer
  4. Use the memory through the pointer, as if it was the correct type
  5. When finished, free the memory

Positioning of struct elements​

  • Like arrays, the positions of the members inside a struct are known
  • Elements will be placed sequentially in memory, in the order they are defined in the structure

#pragma​

  • structs may get empty space in them
  • Can usually tell compiler to pack structs, ignoring speed
  • #pragma means a compiler/os specific pre-processor directive

Unions​

  • Very similar to structs except that the data members are in the same place
  • In unions data members all have the same address
  • Elements of unions are in the SAME place and may be different sizes
  • Unions is as big as the biggest thing inside of it
  • sizeof() is of the largest member

Bit fields​

  • Within structs you can specify fields with size less than a byte

C++ Things​

  • C++ structs can also act as full classes
  • Structs default to public, whereas classes are private

Use new to create a new object of type

New​

  • Knows how big the object is
  • Creates an object and returns a pointer
  • How to create the object in memory
  • Calls the constructor
  • Throws an exception if it fails

delete - destroys an object

Both have a [] version for creating and destroy arrays