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C++ const实现原理

2016-03-21 17:36 246 查看
const 用于修饰常量时，分两种情况

1. 被修饰的常量是基本类型

此时，程序在编译时，就将变量用常量来替换了。

实现机制：这些在编译期间完成，对于内置类型，如int，编译器可能使用常数直接替换掉对此变量的引用。而对于结构体不一定。

看下面的例子：


const int j=100; int *p=const_cast<int*>(&j); *p=200; cout<<j<<endl; 输出为什么是100呢？

cout<<*p<<endl; //输出是改过的200

编译器在优化代码时把cout<<j直接优化成cout<<100了，所以虽然p和&j的值一样，但cout<<j不再通过访问j的地址输出。（反汇编时也有看到直接把数字压栈push
100 ）

这是因为，const型在压栈时，是使用的直接的数，就有点像C的#define
a 100

ps：

基本数据类型如下:

NameDescriptionSize*Range*
char
Character or small integer.1bytesigned: -128 to 127

unsigned: 0 to 255
short int
(
short
)		Short Integer.2bytessigned: -32768 to 32767

unsigned: 0 to 65535
int
Integer.4bytessigned: -2147483648 to 2147483647

unsigned: 0 to 4294967295
long int
(
long
)		Long integer.4bytessigned: -2147483648 to 2147483647

unsigned: 0 to 4294967295
bool
Boolean value. It can take one of two values: true or false.1byte
true
or 
false
float
Floating point number.4bytes+/- 3.4e +/- 38 (~7 digits)
double
Double precision floating point number.8bytes+/- 1.7e +/- 308 (~15 digits)
long double
Long double precision floating point number.8bytes+/- 1.7e +/- 308 (~15 digits)
wchar_t
Wide character.2 or 4 bytes1 wide character
2. const 变量修饰的是非基本类型。

此时，程序编译时，不知道该用什么值替换再编译；所以，将会用一块内存地址替换，然后再编译。

#include <iostream>
using namespace std;
struct A
{
int i;
char ch;
A()
{
i = 100;
ch = 'S';
}
};

int main()
{
const A a;
const int i = 200;
int *p1 = (int*)&a.i;
int *p2 = (int*)&i;
*p1 = 1;
*p2 = 2;
//a.i = 200; //报错，左值不能为const
cout << a.i << " " << a.ch << endl;
cout << i << endl;
return 0;
}


运行结果：

1 S 200

const用于修饰函数也包含两种情况

1. const在函数名前。

此种情况下表示，该函数返回值的类型使一个const类型。

<span style="font-family: FangSong_GB2312;"><u>例如函数
</u></span><span style="font-family: FangSong_GB2312;"><span style="white-space: pre;">		</span>const char * GetString(void);
</span><span style="font-family: FangSong_GB2312;"><span style="white-space: pre;">	</span>如下语句将出现编译错误：</span><span style="font-family: FangSong_GB2312;">
<span style="white-space: pre;">		</span>char*str = GetString();
</span><span style="font-family: FangSong_GB2312;"><span style="white-space: pre;">	</span>正确的用法是</span><span style="font-family: FangSong_GB2312;">
<span style="white-space: pre;">		</span>const char *str =GetString();</span>


2. const在成员函数名之后

此种情况下表示，该函数中将不会对改类的成员变量做任何改变。

<span style="font-family: FangSong_GB2312; color: rgb(255, 0, 0);"><span style="white-space: pre;">	</span>任何不会修改数据成员的函数都应该声明为const</span><span style="font-family: FangSong_GB2312; color: rgb(255, 0, 0);">类型。</span><span style="font-family: FangSong_GB2312;">如果在编写</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">成员函数时，不慎修改了数据成员，或者调用了其它非</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">成员函数，编译器将指出错误，这无疑会提高程序的健壮性。以下程序中，类</span><span style="font-family: FangSong_GB2312;">stack</span><span style="font-family: FangSong_GB2312;">的成员函数</span><span style="font-family: FangSong_GB2312;">GetCount</span><span style="font-family: FangSong_GB2312;">仅用于计数，从逻辑上讲</span><span style="font-family: FangSong_GB2312;">GetCount</span><span style="font-family: FangSong_GB2312;">应当为</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">函数。编译器将指出</span><span style="font-family: FangSong_GB2312;">GetCount</span><span style="font-family: FangSong_GB2312;">函数中的错误。</span><span style="font-family: FangSong_GB2312;">
classStack
{
<span style="white-space: pre;">	</span>public:
<span style="white-space: pre;">		</span>void Push(int elem);
<span style="white-space: pre;">		</span>int Pop(void);
<span style="white-space: pre;">		</span>int GetCount(void) const; // const </span><span style="font-family: FangSong_GB2312;">成员函数</span><span style="font-family: FangSong_GB2312;">
<span style="white-space: pre;">	</span>private:
<span style="white-space: pre;">		</span>intm_num;
<span style="white-space: pre;">		</span>int m_data[100];
};</span>
<span style="font-family: FangSong_GB2312;">
int Stack::GetCount(void)const
{
<span style="white-space: pre;">	</span>++ m_num; // </span><span style="font-family: FangSong_GB2312;">编译错误，企图修改数据成员</span><span style="font-family: FangSong_GB2312;">m_num
<span style="white-space: pre;">	</span>Pop();// </span><span style="font-family: FangSong_GB2312;">编译错误，企图调用非</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">函数</span><span style="font-family: FangSong_GB2312;">
<span style="white-space: pre;">	</span>return m_num;
}</span>
<span style="font-family: FangSong_GB2312;">
const </span><span style="font-family: FangSong_GB2312;">成员函数的声明看起来怪怪的：</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">关键字只能放在函数声明的尾部，大概是因为其它地方都已经被占用了。</span><span style="font-family: FangSong_GB2312;">
</span><span style="font-family: FangSong_GB2312;">关于</span><span style="font-family: FangSong_GB2312;">Const</span><span style="font-family: FangSong_GB2312;">函数的几点规则：</span><span style="font-family: FangSong_GB2312;">

a.const</span><span style="font-family: FangSong_GB2312;">对象只能访问</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">成员函数</span><span style="font-family: FangSong_GB2312;">,</span><span style="font-family: FangSong_GB2312;">而非</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">对象可以访问任意的成员函数</span><span style="font-family: FangSong_GB2312;">,</span><span style="font-family: FangSong_GB2312;">包括</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">成员函数</span><span style="font-family: FangSong_GB2312;">.
b.const</span><span style="font-family: FangSong_GB2312;">对象的成员是不可修改的</span><span style="font-family: FangSong_GB2312;">,</span><span style="font-family: FangSong_GB2312;">然而</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">对象通过指针维护的对象却是可以修改的</span><span style="font-family: FangSong_GB2312;">.
c.const</span><span style="font-family: FangSong_GB2312;">成员函数不可以修改对象的数据</span><span style="font-family: FangSong_GB2312;">,</span><span style="font-family: FangSong_GB2312;">不管对象是否具有</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">性质</span><span style="font-family: FangSong_GB2312;">.</span><span style="font-family: FangSong_GB2312;">它在编译时</span><span style="font-family: FangSong_GB2312;">,</span><span style="font-family: FangSong_GB2312;">以是否修改成员数据为依据</span><span style="font-family: FangSong_GB2312;">,</span><span style="font-family: FangSong_GB2312;">进行检查</span><span style="font-family: FangSong_GB2312;">.
e.</span><span style="font-family: FangSong_GB2312;">然而加上</span><span style="font-family: FangSong_GB2312;">mutable</span><span style="font-family: FangSong_GB2312;">修饰符的数据成员</span><span style="font-family: FangSong_GB2312;">,</span><span style="font-family: FangSong_GB2312;">对于任何情况下通过任何手段都可修改</span><span style="font-family: FangSong_GB2312;">,</span><span style="font-family: FangSong_GB2312;">自然此时的</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">成员函数是可以修改它的</span>
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