C++指针之间的赋值与转换规则总结

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const void *
pVoid_c = pInt;         //ok
pVoid_c = pInt_c;       //ok
pVoid_c = pChar;        //ok
pVoid_c = pChar_c;      //ok
pVoid_c = pDouble;      //ok
pVoid_c = pDouble_c;    //ok
pVoid_c = pA;           //ok
pVoid_c = pA_c;         //ok
pVoid_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;	        //ok
pVoid = pChar;          //ok
pVoid = pDouble;        //ok
pVoid = pA;             //ok
pVoid = 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 '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;     //ok
pVoid = (void*)pChar_c;    //ok
pVoid = (void*)pDouble_c;  //ok
pVoid = (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;      //ok
pInt = (int*)pf;           //ok
pInt = (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;          //ok
pA = (A*)pf;               //ok         
pf = (pFun)pDouble;        //ok
pf = (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;	//ok
pB = (B*)pA;	//ok
pB = (B*)pC;	//ok
pC = (C*)pA;	//ok
pC = (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 convert from '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a
int* 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: 50
cout << *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