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.

int * const x;

A constant pointer to an modifiable integer.

const int * const x;

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

  1. 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 ...

  2. 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 ...

  3. 《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 ...

  4. 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 ...

  5. 【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 ...

  6. (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 ...

  7. (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 ...

  8. 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 ...

  9. 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

  1. 读书笔记_Effective_C++_条款四十四:将与参数无关的代码抽离template

    标题上说“将与参数无关的代码抽离template”,这里的参数既可以指类型,也可以是非类型,我们先来看看非类型的情况. 假定我们要为矩阵写一个类,这个矩阵的行列元素个数相等,是一个方阵,因而我们可以对 ...

  2. Qt 添加资源文件

    *本人乃小白,博文主要用于个人记录,不保证内容准确无误* 我们编写的gui可能需要一些额外的资源(比如贴图用的图片),可用资源文件统一管理.以下以图片为例. 用qt creator 打开工程,为工程新 ...

  3. [原]Ubuntu 14.04编译Android Kernel

    如何编译android kernel参考官方文档:https://source.android.com/source/building-kernels.html   在Ubuntu 14.04上编译a ...

  4. MyBatis知多少(26)MyBatis和Hibernate区别

    iBatis和Hibernate之间有着较大的差异,但两者解决方案很好,因为他们有特定的领域.我个人建议使用MyBatis的,如果: 你想创建自己的SQL,并愿意维持他们. 你的环境是由关系数据模型驱 ...

  5. [IR] Probabilistic Model

    If user has told us some relevant and some irrelevant documents, then we can proceed to build a prob ...

  6. SQL Server 2008 各种DateTime的取值范围

    SQL Server 2008除了DateTime和SmallDateTime之外,又新增了四种时间类型,分别为:date,time,datetime2和datetimeoffset. 各时间类型范围 ...

  7. Lo-Dash – 替代 Underscore 的优秀 JS 工具库

    前端开发人员大都喜欢 Underscore,它的工具函数很实用,用法简单.这里给大家推荐另外一个功能更全面的 JavaScript 工具——Lo-Dash,帮助你更好的开发网站和 Web 应用程序. ...

  8. .NET框架面向对象分层的个人想理

    简单.层次清晰不要过度优化,接口这玩意儿就是个双刃剑,玩好了解藕,玩不好自找麻烦,好的代码永远都是傻瓜都能看懂的. 总结成以下几条: 公用层 代码公用并且与第三方DLL和业务逻辑无关的 独立出来 逻辑 ...

  9. 【HTML5】Canvas 实现放大镜效果

    图片放大镜 效果 在线演示    源码 原理 首先选择图片的一块区域,然后将这块区域放大,然后再绘制到原先的图片上,保证两块区域的中心点一致, 如下图所示: 初始化 <canvas id=&qu ...

  10. [JS] JavaScript框架(2) D3

    D3(Data-Driven Documents)是一个用于网页作图.生成互动图形的JavaScript函数库. 官网:http://d3js.org/ 下载: cdn:<script src= ...