Objective-C编程：从C++到Objective-C的转换

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"从C++到Objective-C的转换指南，作者Pierre Chatelier，由Aaron Vegh翻译成英文，提供了从C++编程语言过渡到Objective-C编程语言的详细概述。文档包含了Objective-C的历史、语法概览、关键词、注释、代码与声明的混合、新类型和值、消息传递、类和对象、协议、类别等内容，适用于想要了解或学习Objective-C的C++程序员。" 本文档主要介绍了C++程序员如何过渡到Objective-C编程，Objective-C是苹果操作系统如Mac OS X和iOS的主要开发语言，它在C++的基础上添加了面向对象特性，并结合了Apple的Cocoa框架。 1. Objective-C和Cocoa - Objective-C起源于80年代，由Stepanov和Musser的C++语言影响，但更注重动态性。它最初是为NeXTSTEP操作系统设计的，后来成为Apple开发平台的核心语言。 - Objective-C2.0是Objective-C的一个重要更新，引入了一些现代语言特性，比如属性（properties）和垃圾回收（garbage collection），并优化了语法。 2. 语法概览 - 关键词：Objective-C的关键字包括"class"、"interface"、"implementation"等，它们用于定义类和实现方法。 - 注释：Objective-C支持多行和单行注释，与C++相似。 - 代码与声明的混合：Objective-C允许在同一个源文件中混合代码和声明，使得代码更紧凑，但可能对初学者来说较难理解。 - 新类型和值：Objective-C引入了id类型，可以代表任何Objective-C对象，以及nil，表示对象指针的空值。 3. 消息传递 - Objective-C使用消息传递机制进行方法调用，类似于C++的函数调用，但更灵活，如`[receiver message]`。 4. 类和对象 - Objective-C是面向对象的语言，类是对象的蓝图，对象则是类的实例。 - 类定义包括接口（Interface）和实现（Implementation）两个部分。 5. 协议 - 协议（Protocol）是Objective-C中的一个重要概念，类似于Java和C#的接口，定义了一组方法签名，可以被多个类遵循。 6. 类别（Categories） - 类别允许向已有的类添加方法，而无需继承，是Objective-C中实现代码复用的独特方式。这篇文档通过对比C++和Objective-C的差异，帮助开发者理解Objective-C的核心特性，以便更好地适应Apple的开发环境。由于Objective-C是Cocoa框架的基础，掌握它对于开发iOS和Mac应用至关重要。

3.2.3 public, private, protected

One major feature of the object model is data encapsulation, which limits the visibility of data to

some parts of the code, in order to ensure its integrity.

Objective-C

class Foo

{

public:

int x;

int apple();

protected:

int y;

int pear();

private:

int z;

int banana();

};

@interface Foo : NSObject

{

@public

int x;

@protected:

int y;

@private:

int z;

}

-(int) apple;

-(int) pear;

-(int) banana;

@end

In C

, attributes and methods can belong to a public, protected or private scope. The

default mode is private.

In Objective-C, only the instance data can be public, protected or private, and the default

visibility is protected. Methods can only be public. However, it is possible to mimic the private

mode, by implementing some methods in the @implementation, without declaring them in the

@interface, or using the notion of class category (cf. section 4.5 on page 25). It does not prevent

methods from being called, but they are less exposed. Implementing a method without a previ-

ous declaration is a special property of Objective-C and has a speciﬁc purpose, as explained in

Section 4.3.2 on page 21.

Inheritance cannot be tagged public, protected or private. The only way is public. Inher-

itance in Objective-C looks more like Java than C

(section 4 on page 21).

3.2.4 static attributes

It’s not possible in Objective-C to declare a class data attribute (static as in C

). However, it

is possible to do the same thing in a diﬀerent way: use a global variable in the implementation ﬁle

(optionally with the static C keyword to limit the scope). The class can then use accessors on

it (with class methods or normal methods), and its initialization can be done in the initialize

method of the class (cf. section 5.1.10 on page 34).

3.3 Methods

The syntax for a method in Objective-C is diﬀerent from the syntax for common C functions. This

section aims to describe this syntax and adds some information on the underlying message sending

principle.

3.3.1 Prototype and call, instance methods, class methods

• A method is preﬁxed by “−” if it is an instance method (common case), or by “+” if it is

a class method (static in C

). This symbol is in no way linked to the UML notation for

public or private. Methods are always public in Objective-C;

• the types for the return value, or for the parameters, are enclosed by parentheses;

• the parameters are separated by a colon “:”;

• the parameters can be associated with a label, a name speciﬁed just before “:”; the label

is then part of the name of the method and modiﬁes it. That makes calls to this

function particularly readable. In fact, the label usage should be systematic. Note that the

ﬁrst parameter cannot have a label – indeed, its label is already the name of the method;

• a method name can be the same as an attribute name, without conﬂict. This is very useful

for getters (cf. section 6.4.8 on page 46).

//prototype

void Array::insertObject(void *anObject, unsigned int atIndex)

//use with a "shelf" instance of the Array class and a "book" object

shelf.insertObject(book, 2);

Objective-C

Without label (direct translation from C

)

//prototype

//The method is named "insertObject::", the colon being used to separate

//the parameters (it is not a scope resolution operator like in C++)

-(void) insertObject:(id)anObject:(unsigned int)index

//use with a "shelf" instance of the Array class and a "book" object

[shelf insertObject:book:2];

With label

//prototype. The "index" parameter is now labeled "atIndex"

//The method is now named "insertObject:atIndex:"

//The call can easily be read like a sentence

-(void) insertObject:(id)anObject atIndex:(unsigned int)index

//use with a "shelf" instance of the Array class and a "book" object

[shelf insertObject:book:2]; //Error !

[shelf insertObject:book atIndex:2]; //OK

Please note that the syntax with square brackets should not be read as calling the method

insertObject of object “shelf” but rather sending the message insertObject to object “shelf”.

This is what makes Objective-C. One can send any message to any target. If it is not able to

handle the message, it can ignore it (an exception is raised but the program does not abort). If,

at the time it receives the message, a target is able to handle it, then the matching method will

be called. The compiler is able to raise a warning if a message is sent to a class for which it is

known that no matching method exists. This is not considered as an error thanks to the forwarding

facility (cf. section 3.4.3 on page 19). If a target is only known under the type id, no warning will

be made at compile time; a potential error would be raised at run-time.

3.3.2 this, self and super

There are two particular targets for a message: self and super. self is the current object (like

this in C

), super is the parent class. The keyword this does not exist in Objective-C. It is

replaced by self.

NB: self is not a real keyword, it is a hidden parameter that each method receives, the value

of which is the current object. Its value can be changed, unlike C

’s this keyword. However,

this is only useful in constructors (cf. section 5.1 on page 27).

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