C: const and static keywords
原文:http://www.noxeos.com/2011/07/29/c-const-static-keywords/
C: const and static keywords
Ok, once and for all, I’ll try to clarify to meaning of the ‘const’ and ‘static’ keywords in C (it applies to Objective-C and C++ too).
I’m just tired of questions about this on StackOverflow.
Objective-C programmers usually don’t think about C code while coding. I personally think this is a big mistake (it can sometimes apply to C++ programmers too).
Objective-C is just a layer over C. So in order to be a good Objective-C programmer, you HAVE to know at least a few things about the C language.
I don’t know why C has such a bad reputation for Objective-C coders, especially on iOS. And it’s very surprising to see the lack of knowledge of some Objective-C developers.
So once and for all:
If you want to code in Objective-C, learn at least the following C topics:
- Language keywords and their meanings
- Pointers and pointer arithmetic
- C standard library
Those are the (very) basics.
Objective-C is a very nice object-oriented language, with amazing runtime capabilities. That’s true.
But it doesn’t mean you can bypass completely the C language.
A lot of time, you’ll save a lot of processor time and memory, just by knowing a few things about the C language, rather than relying on the apparent simplicity of the Objective-C language.
But that’s a different story. Now back on our keywords…
const
First of all, the ‘const’ keyword.
Ok, it means ‘constant’… So:
const int x = 42;
declares a constant integer variable. It means it’s value can’t be modified. It’s value is initially assigned to 42.
If you try to change its value later, the compiler will issue a warning, or an error, depending on your compiler settings.
So the following statement is invalid:
const int x = 42;
x = 43;
That’s pretty easy to understand.
The problem comes with pointers.
Let’s take a look at the following code:
char * str = "hello, world";
It declares a ‘char’ pointer. Ok… But then what about this:
char * const str = "hello, world";
or
const char * str = "hello, world";
Now read carefully.
The first one declares a constant pointer to a char.
It means the the characters of the string can be modified, but not the pointer value.
So the variable ‘str’ cannot be assigned to another pointer.
For instance, this is invalid:
char * hello = "hello, universe";
char * const str = "hello, world";
str = hello;
as your a modifying the pointer value (not the string value).
This is valid:
char * const str = strdup( "hello, world" );
str[ 0 ] = 'a';
The ‘str’ variable will then contain ‘hello, world’. Remember: the pointer can’t be modified, the value that is pointed can be.
It’s the exact opposite with the following notation:
const char * str = "hello, world";
Here, you can assign the pointer to another variable, but you can’t change the value.
The ‘const’ keyword is contextual, in a way, when using pointers. It can apply to the pointer itself, or to the value pointed.
So, in order to resume:
const int * x;
A modifiable pointer to a constant integer.
A constant pointer to an modifiable integer.
A constant pointer to a constant integer.
static
The static keyword can have two meanings.
First of all, it can be declared inside a function.
Let's take a look at this example:
#include <stdio.h>
void foo( void );
void bar( void );
void foo( void )
{
int x = 0;
printf( "X - foo: %i\n", x );
x++;
}
void bar( void )
{
static int x = 0;
printf( "X - bar: %i\n", x );
x++;
}
int main( void )
{
foo();
foo();
foo();
bar();
bar();
bar();
return 0;
}
The output will be:
X - foo: 0
X - foo: 0
X - foo: 0
X - bar: 0
X - bar: 1
X - bar: 2
Because a simple local variable, as in the 'foo' function, only exists when the function is called. It's destroyed (to be simple) when the function exits.
So for the 'foo' function, the variable is created each time the function is called, with a value of '0'. The value is printed, then incremented.
The function then exit, and the variable is destroyed.
But in the 'bar' function, the variable is declared as static. It means the value will persist across function calls.
It's initialized the first time the function is called, but only at that time. Once it has been initialized, it just exist, so its value will be taken for the next function calls.
Now the 'static' keyword as a completely different meaning when used in a variable declared outside of a function (in the global scope).
It means that the variable will be «file scoped». In other words, the variable, which is global, will be accessible only from the scope of the file which declared it. It won't be accessible from other files.
It's just a way to create global private variable.
For instance, imagine a file called 'foo.c':
int x = 42;
static int y = 42;
From a 'bar.c' file, you'll be able to access the 'x' symbol, if both files are linked together. But you won't be able to access the 'y' symbol, as it's decaled as 'static'.
It means that the symbol for the 'y' variable won't be exported by the linker, when the symbol for the 'x' variable will be.
In other words, you'll be able to access the 'y' global variable only from function declared in the 'foo.c' file. The 'x' variable will be also accessible from other files.
Of course, the 'static' keyword can be combined with const.
For instance:
static const int * const y;
A constant pointer to a constant integer, that will be accessible only from the file which declared it.
C: const and static keywords的更多相关文章
- const extern static 终极指南
const extern static 终极指南 不管是从事哪种语言的开发工作,const extern static 这三个关键字的用法和原理都是我们必须明白的.本文将对此做出非常详细的讲解. co ...
- const、static和extern的正确使用方式
我们在看一些大牛的第三方时,里面会出现很多const.static和extern,尤其是const和static,const和extern的结合使用,直接令很多小伙伴懵逼了,今天就详细讲解一下这三个关 ...
- 《OOC》笔记(1)——C语言const、static和extern的用法
<OOC>笔记(1)——C语言const.static和extern的用法 C语言中const关键字用法不少,我只喜欢两种用法.一是用于修饰函数形参,二是用于修饰全局变量和局部变量. 用c ...
- 到底是 const 还是 static readonly
真的一样? const 和 static readonly 常在程序中用来声明常量,调用方法也没有什么不同,他们真的一样吗?我们可以做个试验. 程序集内的常量 现在我们建立一个程序,里面有一个MyCl ...
- 【转】const和static readonly
我们都知道,const和static readonly的确很像:通过类名而不是对象名进行访问,在程序中只读等等.在多数情况下可以混用.二者本质的区别在于,const的值是在编译期间确定的,因此只能在声 ...
- (C#) What is the difference between "const" and "static readonly" ?
const int a must be initialized initialization must be at compile time readonly int a can use defaul ...
- (转) C++ static、const和static const 以及它们的初始化
const定义的常量在超出其作用域之后其空间会被释放,而static定义的静态常量在函数执行后不会释放其存储空间. static表示的是静态的.类的静态成员函数.静态成员变量是和类相关的,而不是和类的 ...
- C# const与static的理解
C# const与static的理解 static readonly与 const变量,作用是一样的,无论访问修饰符是不是public,还是其它(private. protected.interna ...
- c#中const、static、readonly的区别
1. const与readonly const ,其修饰的字段只能在自身声明时初始化. Readonly 是只读变量,属于运行时变量,可以在类初始化的时候改变它的值.该类型的字段,可以在声明或构造函数 ...
随机推荐
- 看上去很美 国内CDN现状与美国对比
CDN的理想与现实 多年以前,当<Kingdom of Heaven>这部史诗电影发行的时候,中国的影迷使用电驴和BT来寻找种子,而那个时候,高清也才刚刚进入电影领域,我的同事不惜用自家的 ...
- 发现新大陆-JMX
今天接触到这个东西,觉得好有趣,可以用很多第三方的显示层jar包直接在UI界面上操作指定的java对象,网上将这个东西的也挺多的,我个人觉得这个比webServer还强大了.webserver只是公布 ...
- [IR] Evaluation
无序检索结果的评价方法: Precision P = tp/(tp + fp) Recall R = tp/(tp + fn) Accuracy = (tp + tn) ...
- Linux sed Examples--转载
原文地址:https://www.systemcodegeeks.com/shell-scripting/bash/linux-sed-examples/?ref=dzone Sed is basic ...
- JS微信分享不好写?来封装一下
微信开发这块,作为开发工程师来说,一般是避免不了的,也好像发现一些朋友写微信分享都是在每个页面一大把一大把的代码. 代码冗余,即便是复制过来再改也很麻烦. 之前自己封装了一下js,今天来分享一下,希望 ...
- 编码神器——Sublime Text 包管理工具及扩展大全
Sublime Text 是程序员们公认的编码神奇,拥有漂亮的用户界面和强大的功能,例如代码缩略图,多重选择,快捷命令等.还可自定义键绑定,菜单和工具栏.Sublime Text 的主要功能包括:拼写 ...
- 深入浅出 JavaScript 中的 this
在 Java 等面向对象的语言中,this 关键字的含义是明确且具体的,即指代当前对象.一般在编译期确定下来,或称为编译期绑定.而在 JavaScript 中,this 是动态绑定,或称为运行期绑定的 ...
- html5中的大纲
html5中的大纲 前言: 在html5中我们可以使用结构元素来编排一份大纲,这样我们就可以通过这个网页的大纲来了解网页中有哪些内容,网页中以什么样的形式来组织这些内容有更清楚的认识. 1.html5 ...
- CentOS6.5菜鸟之旅:文件权限详解
一.前言 Linux下所有资源.设备均被视作文件来操作,而文件权限则是决定用户可各文件操作的范围,无论是平时使用Linux,还是写程序均涉及这方面.以下为个人学习的整理,供以后查阅. 二. 三种权限 ...
- WebApi 集成 Swagger
1. Swagger(俗称:丝袜哥)是什么东西? Swagger 是一个规范和完整的框架,用于生成.描述.调用和可视化 RESTful 风格的 Web 服务.总体目标是使客户端和文件系统作为服务器以同 ...