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解析C++引用

更新時間：2021年06月09日 10:20:39 作者：lsgxeva

引用是C++引入的新語言特性，是C++常用的一個重要內(nèi)容之一。在工作中發(fā)現(xiàn)，許多人使用它僅僅是想當(dāng)然，在某些微妙的場合，很容易出錯，究其原由，大多因為沒有搞清本源。在本篇中將對引用進(jìn)行詳細(xì)討論，希望對大家更好地理解和使用引用起到拋磚引玉的作用

引言

我選擇寫C++中的引用是因為我感覺大多數(shù)人誤解了引用。而我之所以有這個感受是因為我主持過很多C++的面試，并且我很少從面試者中得到關(guān)于C++引用的正確答案。

那么c++中引用到底意味這什么呢？通常一個引用讓人想到是一個引用的變量的別名，而我討厭將c++中引用定義為變量的別名。這篇文章中，我將盡量解釋清楚，c++中根本就沒有什么叫做別名的東東。

背景

在c/c++中，訪問一個變量只能通過兩種方式被訪問，傳遞，或者查詢。這兩種方式是：

1.通過值訪問/傳遞變量

2.通過地址訪問/傳遞變量–這種方法就是指針

除此之外沒有第三種訪問和傳遞變量值的方法。引用變量也就是個指針變量，它也擁有內(nèi)存空間。最關(guān)鍵的是引用是一種會被編譯器自動解引用的指針。很難相信么？

下面是一段使用引用的簡單c++代碼

#include <iostream.h>  
int main()  
{  
    int i = 10;   // A simple integer variable  
    int &j = i;   // A Reference to the variable i  
    j++;   // Incrementing j will increment both i and j.  
    // check by printing values of i and j  
    cout<<  i  <<  j  <<endl; // should print 11 11  
    // Now try to print the address of both variables i and j  
    cout<<  &i  <<  &j  <<endl;  
    // surprisingly both print the same address and make us feel that they are  
    // alias to the same memory location.  
    // In example below we will see what is the reality  
    return 0;  
}

引用其實就是c++中的常量指針。表達(dá)式int &i = j;將會被編譯器轉(zhuǎn)化成int *const i = &j;而引用之所以要初始化是因為const類型變量必須初始化，這個指針也必須有所指。下面我們再次聚焦到上面這段代碼，并使用編譯器的那套語法將引用替換掉。

#include <iostream.h>  
int main()  
{  
    int i = 10;            // A simple integer variable  
    int *const j = &i;     // A Reference to the variable i  
    (*j)++;                // Incrementing j. Since reference variables are   
                          // automatically dereferenced by compiler  
    // check by printing values of i and j  
    cout<<  i  <<  *j  <<endl; // should print 11 11  
    // A * is appended before j because it used to be reference variable  
    // and it should get automatically dereferenced.  
    return 0;  
}

讀者一定很奇怪為什么我上面這段代碼會跳過打印地址這步。這里需要一些解釋。因為引用變量時會被編譯器自動解引用的，那么一個諸如cout << &j << endl;的語句，編譯器就會將其轉(zhuǎn)化成語句cout << &*j << endl;現(xiàn)在&*會相互抵消，這句話變的毫無意義，而cout打印的j值就是i的地址，因為其定義語句為int *const j = &i;

所以語句cout << &i << &j << endl;變成了cout << &i << &*j << endl;這兩種情況都是打印輸出i的地址。這就是當(dāng)我們打印普通變量和引用變量的時候會輸出相同地址的原因。

下面給出一段復(fù)雜一些的代碼，來看看引用在級聯(lián)(cascading)中是如何運作的。

#include <iostream.h>  
int main()  
{  
    int i = 10; // A Simple Integer variable  
    int &j = i; // A Reference to the variable  
    // Now we can also create a reference to reference variable.   
    int &k = j; // A reference to a reference variable  
    // Similarly we can also create another reference to the reference variable k  
    int &l = k; // A reference to a reference to a reference variable.  
    // Now if we increment any one of them the effect will be visible on all the  
    // variables.  
    // First print original values  
    // The print should be 10,10,10,10  
    cout<<  i  <<  ","  <<  j  <<  ","  <<  k  <<  ","  <<  l  <<endl;  
    // increment variable j  
    j++;   
    // The print should be 11,11,11,11  
    cout<<  i  <<  ","  <<  j  <<  ","  <<  k  <<  ","  <<  l  <<endl;  
    // increment variable k  
    k++;  
    // The print should be 12,12,12,12  
    cout<<  i  <<  ","  <<  j  <<  ","  <<  k  <<  ","  <<  l  <<endl;  
    // increment variable l  
    l++;  
    // The print should be 13,13,13,13  
    cout<<  i  <<  ","  <<  j  <<  ","  <<  k  <<  ","  <<  l  <<endl;  
    return 0;  
}

下面這段代碼是將上面代碼中的引用替換之后代碼，也就是說明我們不依賴編譯器的自動替換功能，手動進(jìn)行替換也能達(dá)到相同的目標(biāo)。

#include <iostream.h>  
int main()  
{  
    int i = 10;         // A Simple Integer variable  
    int *const j = &i;     // A Reference to the variable  
    // The variable j will hold the address of i  
    // Now we can also create a reference to reference variable.   
    int *const k = &*j;     // A reference to a reference variable  
    // The variable k will also hold the address of i because j   
    // is a reference variable and   
    // it gets auto dereferenced. After & and * cancels each other   
    // k will hold the value of  
    // j which it nothing but address of i  
    // Similarly we can also create another reference to the reference variable k  
    int *const l = &*k;     // A reference to a reference to a reference variable.  
    // The variable l will also hold address of i because k holds address of i after  
    // & and * cancels each other.  
    // so we have seen that all the reference variable will actually holds the same  
    // variable address.  
    // Now if we increment any one of them the effect will be visible on all the  
    // variables.  
    // First print original values. The reference variables will have * prefixed because   
    // these variables gets automatically dereferenced.  
    // The print should be 10,10,10,10  
    cout<<  i  <<  ","  <<  *j  <<  ","  <<  *k  <<  ","  <<  *l  <<endl;  
    // increment variable j  
    (*j)++;   
    // The print should be 11,11,11,11  
    cout<<  i  <<  ","  <<  *j  <<  ","  <<  *k  <<  ","  <<  *l  <<endl;  
    // increment variable k  
    (*k)++;  
    // The print should be 12,12,12,12  
    cout<<  i  <<  ","  <<  *j  <<  ","  <<  *k  <<  ","  <<  *l  <<endl;  
    // increment variable l  
    (*l)++;  
    // The print should be 13,13,13,13  
    cout  <<  i  <<  ","  <<  *j  <<  ","  <<  *k  <<  ","  <<  *l  <<endl;  
    return 0;  
}

我們通過下面代碼可以證明c++的引用不是神馬別名，它也會占用內(nèi)存空間的。

#include <iostream.h>  
class Test  
{  
    int &i;   // int *const i;  
    int &j;   // int *const j;  
    int &k;   // int *const k;   
};  
int main()  
{      
    // This will print 12 i.e. size of 3 pointers  
    cout<<  "size of class Test = "  <<   sizeof(class Test)  <<endl;  
    return 0;  
}

結(jié)論

我希望這篇文章能把c++引用的所有東東都解釋清楚，然而我要指出的是c++標(biāo)準(zhǔn)并沒有解釋編譯器如何實現(xiàn)引用的行為。所以實現(xiàn)取決于編譯器，而大多數(shù)情況下就是將引用實現(xiàn)為一個const指針。

引用支持c++虛函數(shù)機(jī)制的代碼

#include <iostream.h>  
class A  
{  
public:  
         virtual void print() { cout<<"A.."<<endl; }  
};  
class B : public A  
{  
public:  
         virtual void print() { cout<<"B.."<<endl; }  
};  
   
class C : public B  
{  
public:  
         virtual void print() { cout<<"C.."<<endl; }  
};  
int main()  
{  
         C c1;  
         A &a1 = c1;  
         a1.print(); // prints C  
         A a2 = c1;  
         a2.print(); // prints A  
         return 0;  
}

上述代碼使用引用支持虛函數(shù)機(jī)制。如果引用僅僅是一個別名，那如何實現(xiàn)虛函數(shù)機(jī)制，而虛函數(shù)機(jī)制所需要的動態(tài)信息只能通過指針才能實現(xiàn)，所以更加說明引用其實就是一個const指針。

以上就是解析C++引用的詳細(xì)內(nèi)容，更多關(guān)于C++引用的資料請關(guān)注腳本之家其它相關(guān)文章！

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