Summary of Manual Memory Management Rules

Here are some rules to remember when working with a project compiled without ARCor garbage collection:

•  If you need to hold onto an object to make sure it doesn’t get destroyed by some-one else, you shouldretainit. Make sure to release the object when you’re done with it.
• Sending a release message does not necessarily destroy an object.When an object’s reference count is decremented to 0, the object is destroyed.The system does this by sending the dealloc message to the object to free its memory.
• Release any objects that you have retained or have created using a copy,mutableCopy,alloc, ornew method.This includes properties that have theretain or copy attribute.You can override dealloc to release your instance variables at the time your object is to be destroyed.
• The autorelease pool provides for the automatic release of objects when the pool itself is drained.The system does this by sending a release message to each object in the pool for each time it was autoreleased. Each object in the autorelease pool whose reference count goes down to 0 is sent a dealloc message to destroy the object.
• If you no longer need an object from within a method but need to return it, send it an autorelease message to mark it for later release.The autorelease message does not affect the reference count of the object.
• When your application terminates, all the memory your objects take up is released,regardless of whether they were in the autorelease pool.
• When you develop Cocoa or iOS applications, autorelease pools will be created and drained throughout execution of the program (this will happen each time an event occurs). In such cases, if you want to ensure that an autoreleased object survives automatic deallocation when the autorelease pool is drained, you need to retain it.All objects that have a reference count greater than the number of autorelease messages they have been sent will survive the release of the pool. 

From:<Programming in Objective-C 4th Edition>