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C++:特殊工具与技术之不可移植的特性（位域）

作者：互联网

为了位置底层编程，C++定义了一些固有的不可移植的特性。所谓不可移植的特性是指因机器而异的特性

当我们将含有不可移植的程序从一台机器转移到另一台机器上时，通常需要重新编写该程序。算术类型的大小在不同机器上不一样，这是一个典型实例
本文介绍的特性有：
位域：从C语言继承而来的特性
volatile：从C语言继承而来的特性
extern：C++新增的链接指示

一、位域

类可以将其（非静态）数据成员定义为位域，在一个位域中含有一定数量的二进制位。当一个程序需要向其他程序或硬件设备传递二进制数据时，通常会用到位域


取地址运算符（&）不能作用域位域，因此任何指针都无法指向类的位域

位域的定义

位域的类型必须是整型或枚举类型
因为带符号位域的行为是由具体实现确定的，所以通常情况下使用无符号类型保存一个位域
位域在表达式的后面用一个冒号跟一个常量表达式，常量表达式用来指示成员所占的二进制位数
例如：

typedef unsigned int Bit;
class File
{
Bit mode : 2; //占2位
Bit modified : 1; //占1位 
Bit prot_owner : 3; //占3位
Bit prot_group : 3; //占3位
Bit prot_world : 3; //占3位
public:
//文件类型以八进制的形式表示
enum modes { READ = 01, WRITE = 02, EXECUTE = 03 };
File &open(modes);
void close();
void write();
bool isRead()const;
void setWrite();
};

位域的压缩： ?如果可能的话，在类的内部连续定义的位域压缩在同一个整型的相邻位，从而提供存储压缩 

?例如在上面的代码中，五个位域可能会存储在同一个unsigned int中

 

 

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标签：压缩,C++,wenjuan,特性,Bit,com,位域,移植
来源： https://www.cnblogs.com/nddtt/p/12990772.html