C++ const &函数名

首先函数名前跟&或者*表示返回的是引用或者指针，加const表示不可更改，

是指该函数的返回值是“常量”，不可修改。通常这样的函数返回一个引用(&)或指针。比如：

int& abc(...){...}

int* xyz(...){...}

以后可以这样使用这两个函数：

abc(...)=100;

*xyz(...)=200;

若加上const之后，就限制这样的赋值。

转载：http://blog.csdn.net/zcf1002797280/article/details/7816977

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">看到</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">关键字，</span><span style="font-family: FangSong_GB2312;">C++</span><span style="font-family: FangSong_GB2312;">程序员首先想到的可能是</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">常量。这可不是良好的条件反射。如果只知道用</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">定义常量，那么相当于把火药仅用于制作鞭炮。</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">更大的魅力是它可以修饰函数的参数、返回值，甚至函数的定义体。</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">const </span><span style="font-family: FangSong_GB2312;">是</span><span style="font-family: FangSong_GB2312;">constant</span><span style="font-family: FangSong_GB2312;">的缩写，“恒定不变”的意思。被</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">修饰的东西都受到强制保护，可以预防意外的变动，能提高程序的健壮性。所以很多</span><span style="font-family: FangSong_GB2312;">C++</span><span style="font-family: FangSong_GB2312;">程序设计书籍建议：“</span><span style="font-family: FangSong_GB2312;">Useconst whenever you need”</span><span style="font-family: FangSong_GB2312;">。</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">
     1.</span><span style="font-family: FangSong_GB2312;"><span style="background: rgb(255, 255, 0);">用const</span><span style="background: rgb(255, 255, 0);">修饰函数的参数

</span></span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">如果参数作输出用，不论它是什么数据类型，也不论它采用“指针传递”还是“引用传递”，都不能加</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">修饰，否则该参数将失去输出功能。<span style="font-family: AR PL UMing HK,serif;"><span style="background: rgb(0, 255, 255);">const</span></span><span style="background: rgb(0, 255, 255);">只能修饰输入参数：</span></span></span>

<span style="font-family: FangSong_GB2312;"><u><span style="font-size:18px;">如果输入参数采用“指针传递”，那么加const修饰可以防止意外地改动该指针，起到保护作用。</span></u></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">例如</span><span style="font-family: FangSong_GB2312;">StringCopy</span><span style="font-family: FangSong_GB2312;">函数：</span></span>

<span style="font-family: FangSong_GB2312;"><span style="font-size:18px;">void StringCopy(char*strDestination, const char *strSource);</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">其中</span><span style="font-family: FangSong_GB2312;">strSource</span><span style="font-family: FangSong_GB2312;">是输入参数，</span><span style="font-family: FangSong_GB2312;">strDestination</span><span style="font-family: FangSong_GB2312;">是输出参数。给</span><span style="font-family: FangSong_GB2312;">strSource</span><span style="font-family: FangSong_GB2312;">加上</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-fami
4000
ly: FangSong_GB2312;">修饰后，<u>如果函数体内的语句试图改动strSource的内容，编译器将指出错误。</u></span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">如果输入参数采用“值传递”，由于函数将自动产生临时变量用于复制该参数，该输入参数本来就无需保护，所以不要加</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">修饰。</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">例如不要将函数</span><span style="font-family: FangSong_GB2312;">voidFunc1(int x) </span><span style="font-family: FangSong_GB2312;">写成</span><span style="font-family: FangSong_GB2312;">voidFunc1(const int x)</span><span style="font-family: FangSong_GB2312;">。同理不要将函数</span><span style="font-family: FangSong_GB2312;">voidFunc2(A a) </span><span style="font-family: FangSong_GB2312;">写成</span><span style="font-family: FangSong_GB2312;">voidFunc2(const A a)</span><span style="font-family: FangSong_GB2312;">。其中</span><span style="font-family: FangSong_GB2312;">A</span><span style="font-family: FangSong_GB2312;">为用户自定义的数据类型。</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">对于非内部数据类型的参数而言，象</span><span style="font-family: FangSong_GB2312;">voidFunc(A a) </span><span style="font-family: FangSong_GB2312;">这样声明的函数注定效率比较底。因为函数体内将产生</span><span style="font-family: FangSong_GB2312;">A</span><span style="font-family: FangSong_GB2312;">类型的临时对象用于复制参数</span><span style="font-family: FangSong_GB2312;">a</span><span style="font-family: FangSong_GB2312;">，而临时对象的构造、复制、析构过程都将消耗时间。</span></span>

<span style="font-family: FangSong_GB2312;"><u><span style="font-size:18px;">为了提高效率，可以将函数声明改为voidFunc(A &a)，因为“引用传递”仅借用一下参数的别名而已，不需要产生临时对象。但是函数voidFunc(A &a) 存在一个缺点：</span></u></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">“</span><span style="font-family: FangSong_GB2312;">引用传递”有可能改变参数</span><span style="font-family: FangSong_GB2312;">a</span><span style="font-family: FangSong_GB2312;">，这是我们不期望的。解决这个问题很容易，加</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">修饰即可，因此函数最终成为</span><span style="font-family: FangSong_GB2312;">voidFunc(const A &a)</span><span style="font-family: FangSong_GB2312;">。</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">以此类推，是否应将</span><span style="font-family: FangSong_GB2312;">voidFunc(int x) </span><span style="font-family: FangSong_GB2312;">改写为</span><span style="font-family: FangSong_GB2312;">voidFunc(const int&x)</span><span style="font-family: FangSong_GB2312;">，以便提高效率？完全没有必要，因为内部数据类型的参数不存在构造、析构的过程，而复制也非常快，“值传递”和“引用传递”的效率几乎相当。</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">问题是如此的缠绵，我只好将“</span><span style="font-family: FangSong_GB2312;">const&”</span><span style="font-family: FangSong_GB2312;">修饰输入参数的用法总结一下。</span></span>

<span style="font-size:18px;"><span style="color: rgb(255, 0, 0); font-family: FangSong_GB2312;">对于非内部数据类型的输入参数，应该将“值传递”的方式改为“const</span><span style="color: rgb(255, 0, 0); font-family: FangSong_GB2312;">引用传递”，目的是提高效率</span><span style="font-family: FangSong_GB2312;">。例如将</span><span style="font-family: FangSong_GB2312;">voidFunc(A a) </span><span style="font-family: FangSong_GB2312;">改为</span><span style="font-family: FangSong_GB2312;">voidFunc(const A &a)</span><span style="font-family: FangSong_GB2312;">。</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">对于内部数据类型的输入参数，不要将“值传递”的方式改为“</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">引用传递”。否则既达不到提高效率的目的，又降低了函数的可理解性。例如</span><span style="font-family: FangSong_GB2312;">voidFunc(int x) </span><span style="font-family: FangSong_GB2312;">不应该改为</span><span style="font-family: FangSong_GB2312;">voidFunc(const int &x)</span><span style="font-family: FangSong_GB2312;">。</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">
     2</span><span style="font-family: FangSong_GB2312;"><span style="background: rgb(255, 255, 0);">用const</span><span style="background: rgb(255, 255, 0);">修饰函数的返回值

</span></span><span style="font-family: FangSong_GB2312;"></span><span style="font-family: FangSong_GB2312;">如果给以“指针传递”方式的函数返回值加</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">修饰，那么函数返回值（即指针）的内容不能被修改，<u>该返回值只能被赋给加const修饰的同类型指针。例如函数
</u></span><span style="font-family: FangSong_GB2312;">constchar * GetString(void);
</span><span style="font-family: FangSong_GB2312;">如下语句将出现编译错误：</span><span style="font-family: FangSong_GB2312;">
char*str = GetString();
</span><span style="font-family: FangSong_GB2312;">正确的用法是</span><span style="font-family: FangSong_GB2312;">
constchar *str =GetString();
</span><span style="font-family: FangSong_GB2312;">如果函数返回值采用“值传递方式”，由于函数会把返回值复制到外部临时的存储单元中，加</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">修饰没有任何价值。</span><span style="font-family: FangSong_GB2312;">
</span><span style="font-family: FangSong_GB2312;">例如不要把函数</span><span style="font-family: FangSong_GB2312;">intGetInt(void) </span><span style="font-family: FangSong_GB2312;">写成</span><span style="font-family: FangSong_GB2312;">constint GetInt(void)</span><span style="font-family: FangSong_GB2312;">。</span><span style="font-family: FangSong_GB2312;">
</span><span style="font-family: FangSong_GB2312;">同理不要把函数</span><span style="font-family: FangSong_GB2312;">AGetA(void) </span><span style="font-family: FangSong_GB2312;">写成</span><span style="font-family: FangSong_GB2312;">constA GetA(void)</span><span style="font-family: FangSong_GB2312;">，其中</span><span style="font-family: FangSong_GB2312;">A</span><span style="font-family: FangSong_GB2312;">为用户自定义的数据类型。</span><span style="font-family: FangSong_GB2312;">
</span><span style="font-family: FangSong_GB2312;">如果返回值不是内部数据类型，将函数</span><span style="font-family: FangSong_GB2312;">AGetA(void) </span><span style="font-family: FangSong_GB2312;">改写为</span><span style="font-family: FangSong_GB2312;">constA &GetA(void)</span><span style="font-family: FangSong_GB2312;">的确能提高效率。但此时千万千万要小心，一定要搞清楚函数究竟是想返回一个对象的“拷贝”还是仅返回“别名”就可以了，否则程序会出错。</span><span style="font-family: FangSong_GB2312;">
</span><span style="font-family: FangSong_GB2312;">函数返回值采用“引用传递”的场合并不多，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。</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">例如：</span><span style="font-family: FangSong_GB2312;">
classA
{
A & operate = (const A &other); // </span><span style="font-family: FangSong_GB2312;">赋值函数</span><span style="font-family: FangSong_GB2312;">
};
Aa, b, c; // a, b, c </span><span style="font-family: FangSong_GB2312;">为</span><span style="font-family: FangSong_GB2312;">A</span><span style="font-family: FangSong_GB2312;">的对象</span><span style="font-family: FangSong_GB2312;">

a= b = c; // </span><span style="font-family: FangSong_GB2312;">正常的链式赋值</span><span style="font-family: FangSong_GB2312;">
(a= b) = c; // </span><span style="font-family: FangSong_GB2312;">不正常的链式赋值，但合法</span><span style="font-family: FangSong_GB2312;">
</span><span style="font-family: FangSong_GB2312;">如果将赋值函数的返回值加</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">修饰，那么该返回值的内容不允许被改动。上例中，语句</span><span style="font-family: FangSong_GB2312;">a= b = c </span><span style="font-family: FangSong_GB2312;">仍然正确，但是语句</span><span style="font-family: FangSong_GB2312;">(a= b) = c </span><span style="font-family: FangSong_GB2312;">则是非法的。<span style="font-family: AR PL UMing HK,serif;">
<span style="background: rgb(255, 255, 0);">
3const </span></span><span style="background: rgb(255, 255, 0);">成员函数</span></span><span style="font-family: FangSong_GB2312;">

</span><span style="color: rgb(255, 0, 0); font-family: FangSong_GB2312;">任何不会修改数据成员的函数都应该声明为const</span><span style="color: rgb(255, 0, 0); font-family: FangSong_GB2312;">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
{
public:
void Push(int elem);
int Pop(void);
intGetCount(void) const; // const </span><span style="font-family: FangSong_GB2312;">成员函数</span><span style="font-family: FangSong_GB2312;">
private:
intm_num;
int m_data[100];
};
int Stack::GetCount(void)const
{
++ 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
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;">
returnm_num;
}
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>
ba09
;<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></span>

<span style="font-family: FangSong_GB2312;"><span style="font-size:18px;"> </span></span>

<span style="font-family: FangSong_GB2312;"><span style="font-size:18px;"> </span></span>

<span style="font-family: FangSong_GB2312;"><span style="font-size:18px;">补充：</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">标题</span><span style="font-family: FangSong_GB2312;">:const</span><span style="font-family: FangSong_GB2312;">放在后面有什么意思？</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">--------------------------------------------------------------------------------
</span><span style="font-family: FangSong_GB2312;">一个函数</span><span style="font-family: FangSong_GB2312;">
AcGePoint3dstartPoint() const;
const</span><span style="font-family: FangSong_GB2312;">放在后面跟前面有区别么</span></span>

<span style="font-size:18px;"><span style="font-family: FangSong_GB2312;">==>
</span><span style="font-family: FangSong_GB2312;">准确的说</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">是修饰</span><span style="font-family: FangSong_GB2312;">this</span><span style="font-family: FangSong_GB2312;">指向的对象的</span><span style="font-family: FangSong_GB2312;">
</span><span style="font-family: FangSong_GB2312;">譬如，我们定义了</span><span style="font-family: FangSong_GB2312;">
classA{
public:
f(int);
};
</span><span style="font-family: FangSong_GB2312;">这里</span><span style="font-family: FangSong_GB2312;">f</span><span style="font-family: FangSong_GB2312;">函数其实有两个参数，第一个是</span><span style="font-family: FangSong_GB2312;">A*const this, </span><span style="font-family: FangSong_GB2312;">另一个才是</span><span style="font-family: FangSong_GB2312;">int</span><span style="font-family: FangSong_GB2312;">类型的参数</span><span style="font-family: FangSong_GB2312;">
</span><span style="font-family: FangSong_GB2312;">如果我们不想</span><span style="font-family: FangSong_GB2312;">f</span><span style="font-family: FangSong_GB2312;">函数改变参数的值，可以把函数原型改为</span><span style="font-family: FangSong_GB2312;">f(constint),</span><span style="font-family: FangSong_GB2312;">但如果我们不允许</span><span style="font-family: FangSong_GB2312;">f</span><span style="font-family: FangSong_GB2312;">改变</span><span style="font-family: FangSong_GB2312;">this</span><span style="font-family: FangSong_GB2312;">指向的对象呢？因为</span><span style="font-family: FangSong_GB2312;">this</span><span style="font-family: FangSong_GB2312;">是隐含参数，</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">没法直接修饰它，就加在函数的后面了，表示</span><span style="font-family: FangSong_GB2312;">this</span><span style="font-family: FangSong_GB2312;">的类型是</span><span style="font-family: FangSong_GB2312;">constA *constthis</span><span style="font-family: FangSong_GB2312;">。</span><span style="font-family: FangSong_GB2312;">
const</span><span style="font-family: FangSong_GB2312;">修饰</span><span style="font-family: FangSong_GB2312;">*this</span><span style="font-family: FangSong_GB2312;">是本质，至于说“表示该成员函数不会修改类的数据。否则会编译报错”之类的说法只是一个现象，根源就是因为</span><span style="font-family: FangSong_GB2312;">*this</span><span style="font-family: FangSong_GB2312;">是</span><span style="font-family: FangSong_GB2312;">const</span><span style="font-family: FangSong_GB2312;">类型的</span></span>