C++/C 重要的知识点 (一) const的作用

 C++有操作内存的能力，但是这也增加了出错的几率。在增加程序的高效和编程的灵活性的同时，我们也要特别的关注程序的健康和稳定。

    有些知识点，但在编程过程中容易被忽略，这种忽略一般是因为坏的编程习惯，并不严谨的编程模式，对于知识点的一知半解。要形成好的编程习惯，写出高质量的代码就必须对一些知识点有清楚的认识。

    下面是我在项目中总结的一些知识点。

 

（一）const的作用

    首先，const修饰的变量代表这个变量分配好内存后就应该立刻初始化，初始化以后这个变量的值就不能改变了。这个我们拿const修饰的的常变量和宏相比。

     1.前者是变量会做格式的检查，宏不会做格式检查，所以前者相对安全，减少出错的可能。

      2.程序时出错时调试的时候，宏通常只会显示具体的值而不会显示宏的名称，这样就增加了定位问题的难度。但是const修饰的常变量则和其他的变量一样会显示出错的变量名，定位问题相对的容易。

 

   其次，我们用一个短小的程序说明const在函数中的作用。

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#include <string> using std :: string ; class CPerson { public: enum Sexual { man , woman ， other }; CPerson ( Sexual sex ); ~ CPerson (){}; void setName ( const string & name ); //1.const代表的是不会改变函数调用的参数（引用）的值 void printData () const ; //2.const代表的是此函数不会改变任何一个成员变量的值 const string & getName (); //3.const代表返回的引用不能被改变 ,但是并不能返回一个临时的变量 private: string m_strName ; const Sexual m_enSex ; public: // 将变量声明为public并不合适 int m_iAge ; float m_iWeight ; float m_iHight ; }; #include "A0624.h" #include <iostream> CPerson :: CPerson ( Sexual sex ) : m_enSex ( sex ){} // CPerson::CPerson(Sexual sex){ m_enSex = sex; } //ERROR “CPerson::m_enSex” const修饰的成员变量: 必须在构造函数基/成员初始值设定项列表中初始化 void CPerson :: setName ( const string & name ) { m_strName = name ; } const string & CPerson :: getName () { return m_strName ; } void CPerson :: printData () const //不仅声明中要带const，在定义的时候也要带const { std :: cout << "the name is " << m_strName << std :: endl ; std :: cout << "the age is " << m_iAge << std :: endl ; std :: cout << "the hight is " << m_iHight << std :: endl ; std :: cout << "the weight is " << m_iWeight << std :: endl ; std :: cout << "the person is " << (( m_enSex == Sexual :: man ) ? "man" : "woman" ) << std :: endl ; } int main ( void ) { CPerson person ( CPerson :: Sexual :: man ); std :: string name ( "BinJey" ); person . setName ( name ); // name的值不会被改变 person . m_iAge = 20 ; person . m_iHight = ( float ) 1.89 ; person . m_iWeight = 178 ; const std :: string & sName = person . getName (); // sName = "change"; // ERRO C2678: 二进制“=”: 没有找到接受“const std::string”类型的左操作数的运算符(或没有可接受的转换) person . printData (); return 0 ; }

 来自CODE的代码片

const.cpp

 

  1.const修饰参数引用，代表的是不会改变函数调用的参数（引用）的值

  2.const放在声明和定义的后面，代表的是此函数不会改变任何一个成员变量的值

  3.const修饰返回的引用，代表返回的引用不能被改变 （函数不能返回一个临时的变量作为引用，引用的本质是地址）

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