为什么我会进入这段代码:
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]|
我认为可以将非常量转换为常量。
最佳答案
这是因为您正试图将 int** 转换为 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”。
const
的使用是一个契约,您不能通过间接引用两个引用来满足这个契约。
来自标准:
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?
因此可以修改 const char
always,只需使用上面的过程。
还有:
char *s1 = 0;
const char *s2 = s1; // OK...
char *a[MAX]; // aka char **
const char * const*ps = a; // no error!
引用自以下链接:
By way of analogy, if you hide a criminal under a lawful disguise, he can then exploit the trust given to that disguise. That's bad.
http://www.parashift.com/c++-faq-lite/constptrptr-conversion.html
与此相关的还有无效转换Derived** → Base**
。如果转换 Derived** → Base**
是合法的,则可以取消引用 Base**
(生成 Base*
),并且Base* 可以指向不同派生类的对象,这可能会导致严重的问题。看看为什么:
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 = ⊂
*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
}
关于c++ - 从 int** 到 const int** 的转换,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/16390294/