C/C++ Basics–>about #define, const

1.
    In C programming, the normal use of #define is to declare an constant vairiable. But it's been replaced by const.
C: #define X 100   ----> C++: const int x=100;

2.
    #define can also define Macro with parameters. But its functionality can also be replaced by inline function with C++
C: #define MAX(a,b) ((a)>(b)?(a):(b))
C++: inline int max(int a,int b){ return a>b?a:b;}

3.
    #define is most used in conditional compling. the conditional compling instruction includes:#if,#else,#endif,#ifdef,#ifndef,#undef.
    it always be used to avoid the head file being referenced more than once.
    eg.
    #ifndef NULL
        #define NULL ((void*)0)
    #endif

    #ifndef _myheadfile_h
        #define _myheadfile_h
    #endif

 

4.const

const有很多作用，主要可以有以下幾種：

1. const int x=10; //定義一個常量 ，該常量分配在靜態儲存區

2 const int *p;//p是一個常量pointer，只能指向常量，因此不能進行左操作

3 int f(const int x) //表示f函數中不能修改x的值

4 int f(int x) const //這種主要用在類的成員函數的修飾上，表示該函數不會修改類的成員變數。