Conversion from int** to const int**

Question

Why do I get in this code:

void foo ( const int ** );

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

    return 0;
}

this error:

Answer 1

it is because you are trying to convert from int** to const int**

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

• int ** is: "a pointer to a pointer to an integer".
• const int ** is: "a pointer to a pointer to a constant integer".

The use of const is a contract and you cannot meet this contract by going through the indirection of two references.

From the standard:

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?

so it would be possible to modify const char always, just use procedure above.

And also:

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

nice cite from the link below:

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

related to this is also invalid conversion Derived** → Base**. If it were legal to convert Derived** → Base**, the Base** could be dereferenced (yielding a Base*), and the Base* could be made to point to an object of a different derived class, which could cause serious problems. See why:

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

consider:

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

You can only add const qualification in a conversion between similiar pointer types if you add const at all levels from the first difference in cv qualification and up.

So, you can convert int** to int const* const*, but not to int const* *. If it were allowed to omit adding const at intermediate levels you would be able to do something like:

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

Answer 3

You're being misled here by C++'s confusing parsing rules for pointers. It might be clearer to look at this way:

typedef const int * ptr_to_const_int;
void foo( ptr_to_const_int *);
int main() {
    int ** v = new int * [10];
    foo(v);

    return 0;
}

What foo()'s parameter list promises is that you'll be passing it a pointer to a (pointer-to-constant-thing). But new int*[10] means "pointer to (pointer-to-not-constant-thing)".

So if foo were defined like this:

foo( const int **p )
{
  (*p); //<-- this is actually of type const int *
}

whereas I think you're expecting that

foo( const int **p )
{
  (*p); //<-- you're expecting this to be of type int *
  p = 0; //<-- and this to throw a compiler error because p is const
}

but it's not p that you're declaring to be constant, it's the thing it points to.

Anyway just use a typedef in this case and everything will be clear and readable.