C++ – Is the following C++ casting correct

ccasting

I read a few posts on the usage of static and dynamic casts specifically from When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used?

I have a doubt regarding the usage of cast in the following manner. Can someone verify the below mentioned code:-

This is upward casting in inheritance hierarchy

template<class Base, class Derived>

inline Handle<Base> STATIC_CAST(Handle<Derived> hd) {

  Handle<Base> hb;

  Derived* dp = hd.get(); // Assume this gives pointer of derived class object

  Base* bp = static_cast<Base*> (dp);

  if(bp) {

     hb = Ptr2Handle(bp); // Assume this give reference to Handle 

  }

  return hb;

}

*Derived is actually the derived class from class Base.

What about downward casting in the following code?

template<class Base, class Derived>

inline Handle<Derived> DYNAMIC_CAST(Handle<Base> hb) {

  Handle<Derived> hd;

  Base* bp = hb.get();

  Derived* dp = dynamic_cast<Derived*> (bp);

  if(dp) {

     hd = Ptr2Handle(dp);

  }

  return hd;

}

What will be the impact if the above two MACROS are passed with Base and Derived class swapped?

Best Answer

If Base is really a base class of Derived, there's absolutely no need for any cast, meaning that static_cast in the above code is absolutely superfluous. It doesn't achieve anything a mere assignment wouldn't do implicitly. Moreover, in upcasts (from derived to base), dynamic_cast is absolutely equivalent to static_cast, meaning that dynamic_cast wouldn't achieve anything new either.

Actually, by placing an explicit static_cast cast in that code, its author enabled forceful "reverse" casts (downcasts). I.e. you can use this code to cast from base classes to derived classes. I don't know whether this was the intent (I doubt it was, judging by the template parameter names), and if it wasn't it might be a good idea to remove the cast entirely, since it is dangerous.

If, despite my doubts, the code was actually supposed to support downcasts, then dynamic_cast might indeed help to catch potential errors. However, keep in mind that dynamic_cast works in downcasts with polymorphic class types only.

Related Solutions

C++ – the difference between #include and #include “filename”

In practice, the difference is in the location where the preprocessor searches for the included file.

For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.

For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.

A more complete description is available in the GCC documentation on search paths.

C++ – What are the differences between a pointer variable and a reference variable in C++

A pointer can be re-assigned:

int x = 5;
int y = 6;
int *p;
p = &x;
p = &y;
*p = 10;
assert(x == 5);
assert(y == 10);

A reference cannot be re-bound, and must be bound at initialization:

int x = 5;
int y = 6;
int &q; // error
int &r = x;

A pointer variable has its own identity: a distinct, visible memory address that can be taken with the unary & operator and a certain amount of space that can be measured with the sizeof operator. Using those operators on a reference returns a value corresponding to whatever the reference is bound to; the reference’s own address and size are invisible. Since the reference assumes the identity of the original variable in this way, it is convenient to think of a reference as another name for the same variable.
```
int x = 0;
int &r = x;
int *p = &x;
int *p2 = &r;

assert(p == p2); // &x == &r
assert(&p != &p2);
```

You can have arbitrarily nested pointers to pointers offering extra levels of indirection. References only offer one level of indirection.

int x = 0;
int y = 0;
int *p = &x;
int *q = &y;
int **pp = &p;

**pp = 2;
pp = &q; // *pp is now q
**pp = 4;

assert(y == 4);
assert(x == 2);

A pointer can be assigned nullptr, whereas a reference must be bound to an existing object. If you try hard enough, you can bind a reference to nullptr, but this is undefined and will not behave consistently.

/* the code below is undefined; your compiler may optimise it
 * differently, emit warnings, or outright refuse to compile it */

int &r = *static_cast<int *>(nullptr);

// prints "null" under GCC 10
std::cout
    << (&r != nullptr
        ? "not null" : "null")
    << std::endl;

bool f(int &r) { return &r != nullptr; }

// prints "not null" under GCC 10
std::cout
    << (f(*static_cast<int *>(nullptr))
        ? "not null" : "null")
    << std::endl;

You can, however, have a reference to a pointer whose value is nullptr.

Pointers can iterate over an array; you can use ++ to go to the next item that a pointer is pointing to, and + 4 to go to the 5th element. This is no matter what size the object is that the pointer points to.
A pointer needs to be dereferenced with * to access the memory location it points to, whereas a reference can be used directly. A pointer to a class/struct uses -> to access its members whereas a reference uses a ..
References cannot be put into an array, whereas pointers can be (Mentioned by user @litb)
Const references can be bound to temporaries. Pointers cannot (not without some indirection):
```
const int &x = int(12); // legal C++
int *y = &int(12); // illegal to take the address of a temporary.
```
This makes const & more convenient to use in argument lists and so forth.

Best Answer

Related Solutions

C++ – the difference between #include and #include “filename”

C++ – What are the differences between a pointer variable and a reference variable in C++

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