从int**转换为const int**

Question

为什么我会加入这段代码:

void foo ( const int ** );

int main() {
    int ** v = new int * [10];
    foo(v);

    return 0;
}

这个错误:

invalid conversion from ‘int**’ to ‘const int**’ [-fpermissive]|

我认为可以从非const转换为const.

Answer 1

这是因为你试图转换 int** to const int**

int ** v = new int * [10]; // v is int**
foo(v); //but foo takes const int**

• int ** 是:"指向整数指针的指针".
• const int ** 是:"指向常量整数的指针".

使用const是一个合同,你不能通过两个引用的间接来满足这个合同.

从标准:

const char c = 'c';
char* pc;
const char** pcc = &pc;   // not allowed (thankfully!)
                ^^^ here the bundit is hidden under const: "I will not modify"
*pcc = &c;                // *pcc is "pointer to const" right? so this is allowed...
*pc = 'C';                // would allow to modify a const object, *pc is char right?

所以const char 总是可以修改,只需使用上面的程序.

并且:

char *s1 = 0;
const char *s2 = s1; // OK...
char *a[MAX]; // aka char **
const char * const*ps = a; // no error!

从下面的链接中引用:

通过类比的方式,如果你在合法的伪装下隐藏罪犯,他就可以利用对伪装的信任.那很糟.

http://www.parashift.com/c++-faq-lite/constptrptr-conversion.html

与此相关的转换也是无效的Derived** ? Base**.如果转换是合法的Derived** ? Base**,则Base**可以取消引用(产生a Base*),并且可以使Base*指向不同派生类的对象,这可能导致严重问题.看看原因:

class Vehicle {
public:
  virtual ~Vehicle() { }
  virtual void startEngine() = 0;
};

class Car : public Vehicle {
public:
  virtual void startEngine();
  virtual void openGasCap();
};

class NuclearSubmarine : public Vehicle {
public:
  virtual void startEngine();
  virtual void fireNuclearMissle();
};

int main()
{
  Car   car;
  Car*  carPtr = &car;
  Car** carPtrPtr = &carPtr;
  Vehicle** vehiclePtrPtr = carPtrPtr;  // This is an error in C++
  NuclearSubmarine  sub;
  NuclearSubmarine* subPtr = ⊂
  *vehiclePtrPtr = subPtr;
  // This last line would have caused carPtr to point to sub !
  carPtr->openGasCap();  // This might call fireNuclearMissle()!
  ...
}

http://www.parashift.com/c++-faq-lite/derivedptrptr-to-baseptrptr.html

考虑:

class Vehicle {
public:
  virtual ~Vehicle() { }
  virtual void startEngine() = 0;
};
class Car : public Vehicle {
public:
  virtual void startEngine(){printf("Car engine brummm\n");}
  virtual void openGasCap(){printf("Car: open gas cap\n");}
    virtual void openGasCap2(){printf("Car: open gas cap2\n");}
      virtual void openGasCap3(){printf("Car: open gas cap3\n");}
            virtual void openGasCap4(){printf("Car: open gas cap4\n");}
}; 
class NuclearSubmarine : public Vehicle {
public:
    int i;
  virtual void startEngine(){printf("Nuclear submarine engine brummm\n");}
    virtual void fireNuclearMissle3(){printf("Nuclear submarine: fire the missle3!\n");}
    virtual void fireNuclearMissle(){printf("Nuclear submarine: fire the missle!\n");}
  virtual void fireNuclearMissle2(){printf("Nuclear submarine: fire the missle2!\n");}
};   
int main(){
  Car   car; Car*  carPtr = &car;
  Car** carPtrPtr = &carPtr;
  //Vehicle** vehiclePtrPtr = carPtrPtr;  // This is an error in C++, But:
  Vehicle** vehiclePtrPtr = reinterpret_cast<Vehicle**>(carPtrPtr);
  NuclearSubmarine  sub; NuclearSubmarine* subPtr = &sub;
  *vehiclePtrPtr = subPtr; // carPtr points to sub !
  carPtr->openGasCap();  // Nuclear submarine: fire the missle3!
  carPtr->openGasCap2();  // Nuclear submarine: fire the missle!
  carPtr->openGasCap3();  // Nuclear submarine: fire the missle2!
  //carPtr->openGasCap4();  // SEG FAULT 
}

Answer 2

如果从cv限定和向上的第一个差异中添加所有级别的const,则只能在类似指针类型之间的转换中添加const限定.

所以,你可以转换int**为int const* const*,但不是int const* *.如果允许在中间级别省略添加const,则可以执行以下操作:

const int c = 29;
int *pi;
const int** ppci = π // only adding const, right
*ppci = &c;
*pi = 0; // changing c ?! but no const_cast in sight