C++指针之间的赋值与转换规则总结

来源：互联网发布：农村淘宝佣金编辑：程序博客网时间：2024/05/26 20:24

Note:以下结论不适用于类的成员函数指针，关于类的成员函数指针会单独讨论。

一、任何类型的指针变量均可直接赋值给const void *

　　任何类型的非const指针变量均可直接赋值给void *

　　const指针变量不可直接赋值给void *，除非通过强制类型转换

class A{}; typedef int (*pFun)(string);  //函数指针 int *pInt;const int *pInt_c;char *pChar;const char *pChar_c;double *pDouble;const double *pDouble_c;A *pA;          //自定义类型指针const A *pA_c;pFun pf;   //函数指针 void* pVoid;const void* pVoid_c; // 1.任何类型的指针变量均可直接赋值给constvoid *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.任何类型的非const指针变量均可直接赋值给void*pVoid = pInt;           //okpVoid = pChar;          //okpVoid = pDouble;        //okpVoid = pA;             //okpVoid = pf;             //ok // 3.const指针变量不可直接赋值给void *,除非通过强制类型转换pVoid = pInt_c;            //error: cannot convert from 'constint *' to 'void *'pVoid = pChar_c;           //error: cannot convert from 'constchar *' to 'void *'pVoid = pDouble_c;         //error: cannot convert from 'constdouble *' 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、任意类型指针变量之间均可以强制类型转换，包括const与非const指针变量之间的强制类型转换。

pInt = (int*)pDouble;      //okpInt = (int*)pf;           //okpInt = (int*)pInt_c;       //ok:由const指针变量转非const指针变量pInt = (int*)pA_c;         //ok:由const指针变量转非const指针变量pA = (A*)pA_c;             //ok:由const指针变量转非const指针变量pA = (A*)pDouble;          //okpA = (A*)pf;               //ok        pf = (pFun)pDouble;        //okpf = (pFun)pA;             //ok

三、有继承关系的自定义类型之间：子类型指针变量可直接赋值给父类型指针变量

父类型指针变量不可直接赋值给子类型指针变量，除非通过强制类型转换

class A{}; class B : public A  // B继承自A{}; class C{}; A* pA;B* pB;C* pC; pA = pB;        //ok: 子类型指针变量可直接赋值给父类型指针变量pB = pA;        //error: 父类型指针变量不可直接赋值给子类型指针变量,除非强制类型转换                //以下适用规则二：pA = (A*)pC;    //okpB = (B*)pA;    //okpB = (B*)pC;    //okpC = (C*)pA;    //okpC = (C*)pB;    //ok

补充：

1、对于类的成员函数指针，以上原则不适用。

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 convertfrom 'FunPtr' to 'AFunPointer'

我们可以这样理解：类的成员函数指针被限定在具体的某个类的作用域中了，他不能和域外的指针之间转换。

2、除去类的成员函数指针，虽然任意类型指针变量之间均可以强制类型转换，也即可以将const指针强转为非const指针。

但是应注意:如果将const指针强转为非const指针而用于原本const的对象，则产生未定义行为(C++语言未对此种情况进行规定)。如：

const int a = 50;   // 定义const变量及常量const int* p = &a;  // const指针变量p指向const变量aint* q = (int*)p;   // const指针强转为非const指针,则非const指针变量q指向const变量a*q = 56;            // 行为未定义,以下输出为VS2008下的输出结果cout << a << endl;      //输出: 50cout << *p << endl;     //输出: 56,很显然,p已经不指向a了cout << *q << endl;     //输出: 56,很显然,q已经不指向a了

3、关于一般函数指针的强制转换，以下当然也是OK的。

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

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