C++到Objective-C的转型指南

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"《From C++ to Objective-C》是一本帮助C++程序员过渡到Objective-C的书籍，由Pierre Chatelier编写， Aaron Vegh负责英文改编。本书详细介绍了Objective-C语言的关键特性和Cocoa框架，提供了对Objective-C语法的概述，并讨论了其历史背景。书中还包含了对Objective-C中关键字、注释、代码与声明混合、新类型和值的解释，如BOOL、YES、NO、nil、Nil、id等。此外，还提到了类名前缀NS的由来以及函数与方法的区别。" 在编程领域，Objective-C是一种面向对象的编程语言，特别用于Apple的iOS和macOS操作系统。它是C语言的超集，因此对于已经熟悉C++的开发者来说，过渡到Objective-C相对较为平滑。《From C++ to Objective-C》这本书正是针对这样的转型需求，提供了详细的学习路径。首先，Objective-C和Cocoa是苹果开发平台上的核心技术栈。Objective-C扩展了C语言，引入了消息传递机制和类的概念，而Cocoa则是一个强大的框架集合，用于构建iOS和macOS应用。理解这两者的关系和交互是成为有效Objective-C开发者的基石。书中的"Objective-C和Cocoa"章节会引导读者了解Objective-C的基础，包括它的历史发展，这有助于理解语言的设计哲学。"A short history of Objective-C"部分将介绍Objective-C是如何从Smalltalk和其他语言中汲取灵感，逐渐演变为今天的样子。 "Syntax overview"部分详细解析了Objective-C的语法特性，如关键词的使用，如何添加注释，以及如何在代码中混合声明和实现。Objective-C允许在同一个文件中声明和定义类，这是与C++的一个显著不同之处。此外，书中还会讲解Objective-C中的新类型，比如BOOL类型用于布尔值（YES和NO），nil和Nil用于指针（前者用于对象指针，后者用于类指针），以及SEL类型，它表示方法选择器，用于在运行时调用方法。 "Classnames: why NS?"这部分探讨了为什么Objective-C的很多类名前都带有“NS”前缀，这源自NeXTSTEP系统的历史，NeXTSTEP是Objective-C早期的重要平台。这个前缀通常表示命名空间，有助于避免命名冲突。最后，作者提到了"Diﬀerencing functions and method"，这是Objective-C与C++的一个关键差异。在Objective-C中，方法调用是通过消息传递完成的，而在C++中，函数调用则是直接的。这种差异影响了代码的编写方式，理解这一点对于从C++转到Objective-C的开发者至关重要。《From C++ to Objective-C》是一本深入浅出的指南，它不仅介绍了Objective-C的基本语法，还涵盖了与C++的对比，帮助开发者快速适应新的编程环境。通过阅读此书，C++程序员可以更好地理解和应用Objective-C，从而在iOS和macOS开发中游刃有余。

In Objective-C, attributes and methods cannot be mixed. The attributes are declared in braces,

the methods follow. Their implementation lies in an @implementation block.

This is a major diﬀerence with C

, since some methods can be implemented without being

exposed in the interface. This is detailed later. Brieﬂy, this is a way to clean up header ﬁles by

removing unnecessary declarations (“private” methods, and silently redeﬁned virtual methods like

destructors). Please see Section 6.3.2 on page 19 for further explanations.

Instance methods are preﬁxed by the minus symbol “-”, and class methods by the plus symbol

“+”(cf. section 5.3.9 on page 17); this symbol has nothing to do with the UML notation and

the meaning public or private. The type of the parameters are enclosed in parenthesis, and the

parameters are separated by the symbol “:”. Please see Section 5.3.1 on the next page for further

explanations on the syntax of prototypes.

In Objective-C, there is no need for a semi-colon at the end of a class declaration. Also note

that the keyword to declare a class is @interface and not @class. The keyword @class is only

used in forward declarations (cf. section 5.2.2 on the current page). Finally, if there is no instance

data in a class, the braces, which would enclose nothing, can be ommitted.

5.2.2 Forward declarations: @class, @protocol

To avoid cyclic dependencies in header ﬁles, the C language supports the forward declaration,

that allows the coder to declare a class when the only required knowledge is its existence and

not its structure. In C

, the keyword class is used; in Objective-C, it is @class. The keyword

@protocol can also be used to anticipate the declaration of a protocol (cf. section 6.4 on page 20).

//In file Foo.h

#ifndef __FOO_H__

#define __FOO_H__

class Bar; //forward declaration

class Foo

{

Bar* bar;

public:

void useBar(void);

};

#endif

//In file Foo.cpp

#include "Foo.h"

#include "Bar.h"

void Foo::useBar(void)

{

...

}

Objective-C

//In file Foo.h

@class Bar; //forward declaration

@interface Foo : NSObject

{

Bar* bar;

}

-(void) useBar;

@end

//In file Foo.m

#import "Foo.h"

#import "Bar.h"

@implementation Foo

-(void) useBar

{

...

}

@end

5.2.3 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 6.5 on page 22). Implementing a

method without a previous declaration is a special property of Objective-C and has a speciﬁc

purpose, as explained in Section 6.3.2 on page 19.

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 6 on page 19).

5.2.4 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 7.1.8 on page 28).

5.3 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.

5.3.1 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;

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