[CPP] Reference, new and delete

Reference

• A way to give a new name to an item
• look like normal variables
• act like pointers
• 

The need for references

• Useful if we need to keep the same syntax(but avoiding making a copy)
• Useful as return values, to chain functions together(especially returning *this from member functions to return reference to current object)
• References are necessary for operator overloading(changing the meaning of operators)

warning

Similar problems with references as with pointers
e.g. do not return a reference to a local variable

When the local variable vanishes (e.g. the function ends), the reference refers to something that doesn’t exist.

new and delete

new vs malloc

• new knows how big the object is: no call to sizeof() is needed (unlike malloc() )
• new creates an object ( and returns a pointer): allocates memory (probably in same way as malloc() )
• new knows how to create the object in memory: c++ objects can consists of more than the visible data members
• new calls the constructor (malloc() will not )
• new throws an exception(bad_alloc) if it fails: by default, unless you tell it not to.

What new really does?

When you call new, the compiler generates code to:

• Call operator new (to allocate the memory)
• Create the object: including hidden data(e.g. vpointers)
• Calls the constructor code

delete

delete destroys an object

• It cares about the object type
• Calls the destructor of the class it thinks the thing is (using pointer type) and then frees the memory

new[] and delete[]

• new and delete have a [] version for creating and destroying arrays
• Default constructor is called for the elements
• you must match together
• new and delete
• new [] and delete[]
• malloc() and free()

pointer problems

• The same kind of problems can occurs with new and delete as malloc() and free():
• memory leak(leaking memory-less available): not calling delete on all of the objects or arrays that you new
• dereferencing a pointer after you have freed/deleted the memory it points to
• Calling delete multiple times on same pointer

Remainder

• If you want to create objects in dynamic memory then you must go through new(not malloc() )
• You can use new on basic types(e.g. int): By default they are not initialized