C/C++语言中关于const用法的总结_const set<int>& sources-优快云博客

原文：http://blog.youkuaiyun.com/liyuefeilong/article/details/48900121

一. const的基本功能与用法

1.将限定符声明为只读

使用方法如下，在类型前/后加上关键字const，该变量必须被初始化，否则编译错误；该变量不能被重新赋值，否则也编译错误。
举例：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">50</span>;   <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 编译正确</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> j;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 编译错误</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> k = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
i = k;              <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 编译错误</span>
k = i;              <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 编译正确</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

2.用于修饰函数形参，保护参数使其不被修改

用法1：若形参为const A* a，则不能改变函数所传递的指针内容，这样对指针所指向的内容起到保护作用，这里需要注意的是，该修饰不能改变指针所指向地址所存储的内容，但是指针a所指向的地址可以被改变，具体例子如下：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> Test(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *a)
{
    *a = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;          <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//错误，*a不能被赋值</span>
    a = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10086</span>);  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//正确，为指针a开辟新的空间，并令*a=2</span>
}

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main()
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *a = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10000</span>);
    Test(a);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

用法2：若形参为const A& a，则不能改变函数传递进来的引用对象，从而保护了原对象的属性。

对于自定义的数据类型，用引用传递速度较快，如果不想改变原值，就可以用const来保护参数，如以下例子：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> Test(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> &a) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//保护L7中的a不会被改变</span>
{
    a = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//错误，a不能给常量赋值</span>
}

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main()
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>;
    Test(a);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

事实上对于内置型数据类型（如以上例子中的int类型），用引用传递不会使速度更快。如果是用引用传参，一般是为了改变参数值；如果不想改变参数的值，直接值传递即可，不必使用const修饰。而对于自定义数据类型的输入参数，为了提高速度和效率，应使用“const + 引用传递”代替值传递。例如：

将函数 void Test(A a) 改为-> void Test(const A &a)

3.用于修饰函数返回值

用法1：用const修饰返回值为对象本身（非引用和指针）的情况多用于二目操作符重载函数并产生新对象的时候
举例：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> Rational <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">operator</span>*(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> Rational& lhs, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> Rational& rhs) 
{ 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> Rational(lhs.numerator() * rhs.numerator(), 
lhs.denominator() * rhs.denominator()); 
} 
Rational a,b; 
Radional c; 
(a*b) = c;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//错误</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

用法2：不建议用const修饰函数的返回值类型为某个对象或对某个对象引用的情况。原因如下：如果返回值为某个对象为const（const A test = A 实例）或某个对象的引用为const（const A& test = A实例） ，则返回值具有const属性，则返回实例只能访问类A中的公有（保护）数据成员和const成员函数，并且不允许对其进行赋值操作，这在一般情况下很少用到，具体例子如下：

<code class="hljs haskell has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">A</span>
{
public:
    int y;
    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">A</span><span class="hljs-container" style="box-sizing: border-box;">(<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">int</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">y</span>)</span>:x<span class="hljs-container" style="box-sizing: border-box;">(<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">x</span>)</span>,y<span class="hljs-container" style="box-sizing: border-box;">(<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">y</span>)</span>{};
    void <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Sety</span><span class="hljs-container" style="box-sizing: border-box;">(<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">int</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">y</span>)</span>{this->y = y;}
};

const <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">A</span> <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Test1</span><span class="hljs-container" style="box-sizing: border-box;">(<span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">A</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">a</span>)</span>
{
    return a;
}

const <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">A</span>& <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Test2</span><span class="hljs-container" style="box-sizing: border-box;">(<span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">A</span> &<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">a</span>)</span>
{
    return a;
}

int main<span class="hljs-container" style="box-sizing: border-box;">()</span>
{
    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">A</span> a<span class="hljs-container" style="box-sizing: border-box;">(2)</span>;
    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Test1</span><span class="hljs-container" style="box-sizing: border-box;">(<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">a</span>)</span>.<span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Sety</span><span class="hljs-container" style="box-sizing: border-box;">(3)</span>;//错误，因为<span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Test1</span><span class="hljs-container" style="box-sizing: border-box;">(<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">a</span>)</span>的返回值是个const，不能被<span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Sety</span><span class="hljs-container" style="box-sizing: border-box;">(3)</span>修改
    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Test2</span><span class="hljs-container" style="box-sizing: border-box;">(<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">a</span>)</span>.<span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Sety</span><span class="hljs-container" style="box-sizing: border-box;">(3)</span>;//错误，因为<span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Test1</span><span class="hljs-container" style="box-sizing: border-box;">(<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">a</span>)</span>的返回值是个const，不能被<span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">Sety</span><span class="hljs-container" style="box-sizing: border-box;">(3)</span>修改
    return 0;
}</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li></ul>

用法3：如果给采用“指针传递”方式的函数返回值加const修饰，那么函数返回值（即指针）的内容不能被修改，该返回值只能被赋给加const 修饰的同类型指针。例子如下：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> * GetString(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span>){}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main()
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> *str1=GetString();<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//错误</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> *str2=GetString();<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//正确</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

用法4：函数返回值采用“引用传递”的场合不多，这种方式一般只出现在类的赋值函数中，目的是为了实现链式表达。例子如下：

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">A</span>
{</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 以下赋值函数的返回值加const修饰，该返回值的内容不允许修改</span>
    A &operate = (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> A &other); 
}
A a, b, c;  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// a,b,c为A的对象</span>
a = b = c;      <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 正确</span>
(a = b) = c;    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 错误，a = b的返回值不允许被再赋值</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

4.在类成员函数的函数体后加关键字const

在类成员函数的函数体后加关键字const，形如：void fun() const; 在函数过程中不会修改数据成员。如果在编写const成员函数时，不慎修改了数据成员，或者调用了其他非const成员函数，编译器将报错，这大大提高了程序的健壮性。

如果不是在类的成员函数，没有任何效果，void fun() const;和void func();是一样的。

5.在另一连接文件文件中引用常量

方法：在类型前添加关键字extern const，表示引用的是常量，因此该常量不允许被再次赋值，举例如下：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extern</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i;       <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 正确</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extern</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> j = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>;  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 错误，常量不可以被再次赋值</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

二.const常量与#define的区别

1.const常量有数据类型，而宏常量没有数据类型

宏常量只进行简单的字符替换，没有类型安全检查，并且在字符替换时可能会产生意料不到的错误，如：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">define</span> I = 10</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> &i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>;
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 由于编译器的优化，使得在const long i=10时i不被分配内存</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 而是已10直接代入以后的引用中，以致在以后的代码中没有错误</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//一旦你关闭所有优化措施，即使const long i = 10也会引起后面的编译错误。</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> h = I; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 正确</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> h = i; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 编译警告，可能由于数的截短带来错误赋值</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

2.使用const可以避免不必要的内存分配

从汇编的角度来看，const定义常量只是给出了对应的内存地址，而不是象#define一样给出的是立即数，所以，const定义的常量在程序运行过程中只有一份拷贝，而#define定义的常量在内存中有若干个拷贝。例子如下：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#define k "Hello world!"</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> pk[]=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Hello world!"</span>;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(k);      <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 为k分配了第一次内存</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(pk);     <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 为pk一次分配了内存，以后不再分配</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(k);      <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 为k分配了第二次内存</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(pk);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

三. 使用const的一些注意事项

1.修改const 所修饰的常量值

以下例子中，i为const修饰的变量，可以通过对i进行类型强制转换，将地址赋给一个新的变量，对该新的变量再作修改即可以改变原来const 修饰的常值。

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *p=(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>*)&i;
p=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

2.构造函数不能被声明为const

3.const数据成员的初始化只能在类的构造函数的初始化表中进行

<code class="hljs vala has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">A</span>
{</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a;
    A(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> x):a(x)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//正确</span>
    {
        a = x;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//错误</span>
    }
};</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

4.在参数中使用const应该使用引用或指针，而不是一般的对象实例

合理利用const在成员函数中的三种用法（参数、返回值、函数），一般来说，不要轻易的将函数的返回值类型定为const；另外，除了重载操作符外一般不要将返回值类型定为对某个对象的const引用。

5.对于使用const修饰来指针的情况

对于以下情况，const放在变量声明符的前后位置效果是一样的，这种情况下不允许对指针a 的内容进行更改操作：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *a = &i;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span>*a = &i;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

但是，如果const位于星号的左侧，则const就是用来修饰指针所指向的变量，即该指针指向一个地址，该地址的内容不可变；如果const位于星号的右侧，const就是修饰指针本身，即指针本身是常量：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i;
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 以下一行表示a是一个指针，可以任意指向int常量或者int变量</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 它总是把它所指向的目标当作一个int常量</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 也可以写成int const* a</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *a = &i;
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 以下一行表示a是一个指针常量，</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 初始化的时候必须固定指向一个int变量</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 之后就不能再指向别的地方了</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 但是指针指向的内容可以改变</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> a = &i;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

6.指针本身是常量，而指针所指向的内容不是常量，这种情况下不能对指针本身进行更改操作，如以下例子中a++是错误的：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> a = &i;
a++;  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 错误，a指针本身是常量，不能再指向别的地方</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

7.当指针本身和指针所指向的内容均为常量时

这种情况下可写为：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> a = &i;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

8.const成员函数返回的引用，也是const

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include<iostream></span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> A
{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> x;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">set</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> x){<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>->x = x;}
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// const成员函数返回的引用也是const，a</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 如果把A&前面的const去掉会出错</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 因为返回的是一个const的对象，返回类型却不是const</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 返回的内容和返回的类型不符</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> A& Test1()<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span>
    {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 错误。这是const成员函数的特点</span>
    x = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 不限于*this。不管返回的是什么，哪怕是一个定义为非const的对象，结果也是一样的</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> *<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>;
    }
};

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main()
{
    A a, b;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 正确，虽然返回的是一个const，却用另一个非const来接收</span>
    b = a.Test1();
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 错误，既然是别名，那么别名的类型要与原来的类型相同</span>
    A &c = a.Test1();
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 正确虽然在a.Test1()中a不能改变，但是这里已经出了这个成员函数的作用域</span>
    a.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">set</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>);
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 正确，b接收了a.Test1()返回的数据的内容，但是它不是const</span>
    b.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">set</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>);
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 错误。a.Test1()是一个对象，这个对象是它的返回值</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 虽然没有名字，但是它就是a.Test1()的返回值</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 值是a.Test1()返回的值，类型是a.Test1()返回的类型</span>
    a.Test1().<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">set</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li></ul>

9.mutable将数据声明为可变数据成员

在C++语言中，mutable是使用较少的关键字，它的作用是：如果一个函数被const 修饰，那么它将无法修改其成员变量的，但是如果一个成员变量是被mutable修饰的话，则可以修改。

mutable 可以用来指出，即使成员函数或者类变量为const，其某个成员也可以被修改。反过来说，可变数据成员永远不能成为const，即使它是const对象的成员。

<code class="hljs vala has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">A</span>
{</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> x;
    mutable <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> y;
    A(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b):x(a),y(b){}
};

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main()
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span><span class="hljs-constant" style="box-sizing: border-box;"> A </span>a(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// const对象必须初始化</span>
    a.x = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;         <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 错误</span>
    a.y = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;         <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 正确，mutable修饰使得成员可被修改，即使对象a为const</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}</code>