《Programming with Objective-C》第三章 Working with Objects

If you’re used to using terms like the stack and the heap, a local variable is allocated on the stack, while objects are allocated on the heap.

- (void)f
{
    int a = 2;                    //Stack
    NSString *s = @"Hello";        //Heap
}

函数f中，a指向的内存在栈中，函数退出的时候变量a将不能再被访问，其内存也会被释放；s指向的内存在堆中，函数退出的时候s也不能再被访问，但是s指向的内存可能继续存在。

Factory Method v.s. Abstract Factory

Todo 等查找资料后再补上

Objective-C是一门动态语言

id someObject = @"Hello, World!";
[someObject removeAllObjects];

编译时，someObject是一个id类型，所以编译器不会报错。

运行时，编译器会出现Runtime Error因为NSString对象不能响应removeAllObjects.

NSString *someObject = @"Hello, World!";
[someObject removeAllObjects];

编译时，编译器知道someObject是一个NSString类型，NSString对象不能响应removeAllObjects，所以编译时编译器会报错

等于/不等于

//基本类型
if (someInteger == 42) 
{
    // someInteger has the value 42
}

//比较是否是同一个对象
if (firstPerson == secondPerson)    
{
    // firstPerson is the same object as secondPerson
}

//比较2个对象的内容是否相等
if ([firstPerson isEqual:secondPerson])     
{
    // firstPerson is identical to secondPerson
}

//NSNumber, NSString and NSDate等类型比较大小不能用>、<，应该用compare:方法
if ([someDate compare:anotherDate] == NSOrderedAscending) 
{
    // someDate is earlier than anotherDate
}