C++指標之間的賦值與轉換規則總結

Note:以下結論不適用於類的成員函數指標，關於類的成員函數指標會單獨討論。

一、任何類型的指標變數均可直接賦值給const void *
　　任何類型的非const指標變數均可直接賦值給void *
　　const指標變數不可直接賦值給void *，除非通過強制類型轉換

class A{};typedef int (*pFun)(string);  //函數指標int *pInt;const int *pInt_c;char *pChar;const char *pChar_c;double *pDouble;const double *pDouble_c;A *pA;          //自訂類型指標const A *pA_c;pFun pf;//函數指標void* pVoid;const void* pVoid_c;// 1.任何類型的指標變數均可直接賦值給const void *pVoid_c = pInt;         //okpVoid_c = pInt_c;       //okpVoid_c = pChar;        //okpVoid_c = pChar_c;      //okpVoid_c = pDouble;      //okpVoid_c = pDouble_c;    //okpVoid_c = pA;           //okpVoid_c = pA_c;         //okpVoid_c = pf;           //ok// 2.任何類型的非const指標變數均可直接賦值給void *pVoid = pInt;        //okpVoid = pChar;          //okpVoid = pDouble;        //okpVoid = pA;             //okpVoid = pf;             //ok// 3.const指標變數不可直接賦值給void *,除非通過強制類型轉換pVoid = pInt_c;            //error: cannot convert from 'const int *' to 'void *'pVoid = pChar_c;           //error: cannot convert from 'const char *' to 'void *'pVoid = pDouble_c;         //error: cannot convert from 'const double *' to 'void *'pVoid = pA_c;              //error: cannot convert from 'const A *' to 'void *'pVoid = (void*)pInt_c;     //okpVoid = (void*)pChar_c;    //okpVoid = (void*)pDouble_c;  //okpVoid = (void*)pA_c;       //ok

　　

二、任意類型指標變數之間均可以強制類型轉換，包括const與非const指標變數之間的強制類型轉換。

pInt = (int*)pDouble;      //okpInt = (int*)pf;           //okpInt = (int*)pInt_c;       //ok:由const指標變數轉非const指標變數pInt = (int*)pA_c;         //ok:由const指標變數轉非const指標變數pA = (A*)pA_c;             //ok:由const指標變數轉非const指標變數pA = (A*)pDouble;          //okpA = (A*)pf;               //ok         pf = (pFun)pDouble;        //okpf = (pFun)pA;             //ok

　　

三、有繼承關係的自訂類型之間：子類型指標變數可直接賦值給父類型指標變數

                                            父類型指標變數不可直接賦值給子類型指標變數，除非通過強制類型轉換

class A{};class B : public A  // B繼承自A{};class C{};A* pA;B* pB;C* pC;pA = pB;        //ok: 子類型指標變數可直接賦值給父類型指標變數pB = pA;        //error: 父類型指標變數不可直接賦值給子類型指標變數,除非強制類型轉換                //以下適用規則二：pA = (A*)pC;//okpB = (B*)pA;//okpB = (B*)pC;//okpC = (C*)pA;//okpC = (C*)pB;//ok

　　

補充：

 1、對於類的成員函數指標，以上原則不適用。

class A{};typedef void (A::*AFunPointer)(void);typedef void (*FunPtr)(void);void * pVoid;int * pInt;FunPtr fp;AFunPointer afp;pVoid = afp;      //error: cannot convert from 'AFunPointer' to 'void *'pInt = (int*)afp;     //error: 'type cast' : cannot convert from 'AFunPointer' to 'int *'fp = (FunPtr)afp;     //error: 'type cast' : cannot convert from 'AFunPointer' to 'FunPtr'afp = (AFunPointer)pInt;     //error: 'type cast' : cannot convert from 'int *' to 'AFunPointer'afp = (AFunPointer)pVoid;    //error: 'type cast' : cannot convert from 'void *' to 'AFunPointer'afp = (AFunPointer)fp;     //error: 'type cast' : cannot convert from 'FunPtr' to 'AFunPointer'

我們可以這樣理解：類的成員函數指標被限定在具體的某個類的範圍中了，他不能和域外的指標之間轉換。

 

2、除去類的成員函數指標，雖然任意類型指標變數之間均可以強制類型轉換，也即可以將const指標強轉為非const指標。

    但是應注意:如果將const指標強轉為非const指標而用於原本const的對象，則產生未定義行為(C++語言未對此種情況進行規定)。如：

const int a = 50;   // 定義const變數及常量const int* p = &a;  // const指標變數p指向const變數aint* q = (int*)p;   // const指標強轉為非const指標,則非const指標變數q指向const變數a*q = 56;            // 行為未定義,以下輸出為VS2008下的輸出結果cout << a << endl;      //輸出: 50cout << *p << endl;     //輸出: 56,很顯然,p已經不指向a了cout << *q << endl;     //輸出: 56,很顯然,q已經不指向a了

 

3、關於一般函數指標的強制轉換，以下當然也是OK的。

class A;typedef void (*pFun1)(int, int);typedef int  (*pFun2)(A*, double);pFun1 pf1;pFun2 pf2;pf2 = (pFun2)pf1;    // OK