Query on Static Cast

[The Saint]

Hi folks,
I got a simple doubt. What are the pros and cons of using c style casting in c++ and vc++. I need to know the difference between

ptr = (class*) (expression) and

ptr = static_cast<class*> (expression).

At what risk could I use ptr = (class*) (expression) in c++ or vc++.
ciao

[Andreas Masur]

Casting usually is considered as bad software design. I would not go that far (at least in Windows you are forced to do it by passing values to API functions (although you will not see it most of the time since it is done implicitely...)). Nevertheless if you need to cast use the casts which were introduced with C++. The following is a small description about C-style and C++-casts...

Casting means you change the representation of a variable by changing its type to a different one. In order to type-cast a simple object to another you use the traditional type casting operator. For example, to cast a floating point number of type 'double' to an integer of type 'int':

int i;
double d;

i = (int) d;

or also

i = int (d);

This is quite good for basic types that have standard defined conversions, however this operators can also been indiscriminately applied on classes and pointers to classes. ANSI-C++ standard has defined four new casting operators: 'reinterpret_cast', 'static_cast', 'dynamic_cast' and 'const_cast' in order to control these types of conversions between classes...

reinterpret_cast<new_type>(expression)
dynamic_cast<new_type>(expression)
static_cast<new_type>(expression)
const_cast<new_type>(expression)

reinterpret_cast

'reinterpret_cast' casts a pointer to any other type of pointer. It also allows casting from pointer to an integer type and vice versa.

This operator can cast pointers between non-related classed. The operation results is a simple binary copy of the value from a pointer to the other. The content pointed does not pass any kind of check nor transformation between types.

In the case that the copy is performed from a pointer to an integer, the interpretation of its content is system dependent and therefore any implementation is non portable. A pointer casted to an integer enough large to fully contain it can be casted back to a valid pointer.

class A {};
class B {};

A * a = new A;
B * b = reinterpret_cast<B *>(a);

'reinterpret_cast' treats all pointers exactly as traditional type-casting operators do.


static_cast

'static_cast' allows to perform any casting that can be implicitly performed as well as also the inverse cast (even if this is not allowed implicitly).

Applied to pointers to classes, that is to say that it allows to cast a pointer of a derived class to its base class (this is a valid conversion that can be implicitly performed) and can also perform the inverse: cast a base class to its derivated class.

In this last case the base class that is being casted is not checked to determine wether this is a complete class of the destination type or not.

class Base {};
class Derived : public Base {};

Base *a    = new Base;
Derived *b = static_cast<Derived *>(a);

'static_cast', aside from manipulating pointers to classes, can also be used to perform conversions explicitly defined in classes, as well as to perform standard conversions between fundamental types:

double d = 3.14159265;
int    i = static_cast<int>(d);

dynamic_cast

'dynamic_cast' is exclusively used with pointers and references to objects. It allows any type-casting that can be implicitly performed as well as the inverse one when used with polymorphic classes, however, unlike static_cast, dynamic_cast checks, in this last case, if the operation is valid. That is to say, it checks if the casting is going to return a valid complete object of the requested type.

Checking is performed during run-time execution. If the pointer being casted is not a pointer to a valid complete object of the requested type, the value returned is a 'NULL' pointer.

class Base { virtual dummy() {} };
class Derived : public Base {};

Base* b1 = new Derived;
Base* b2 = new Base;

Derived* d1 = dynamic_cast<Derived *>(b1);          // succeeds
Derived* d2 = dynamic_cast<Derived *>(b2);          // fails: returns 'NULL'

If the type-casting is performed to a reference type and this casting is not possible an exception of type 'bad_cast' is thrown:

class Base { virtual dummy() {} };
class Derived : public Base { };

Base* b1 = new Derived;
Base* b2 = new Base;

Derived d1 = dynamic_cast<Derived &*>(b1);          // succeeds
Derived d2 = dynamic_cast<Derived &*>(b2);          // fails: exception thrown

const_cast

This type of casting manipulates the const attribute of the passed object, either to be set or removed:

class C {};

const C *a = new C;

C *b = const_cast<C *>(a);

Neither of the other three new cast operators can modify the constness of an object.

[Graham]

One slight addition to Andreas's excellent answer: const_cast can also cast the "volatile" qualifier in addition to the "const" qualifier. (It ought to be called cv_cast, I suppose).

Also one other thought on C-style versus C++-style: as Andreas mentioned, excessive casting probably indicates a fundamental flaw in your code. When that flaw is fixed, and you're searching your code for all the casts that can now be removed, think on which style you'd rather look for....

[The Saint]

Hi mate,
Thank you both for the excellent information. I had actually asked this question in the following scenario;
My code is written in VC++. Fo using GetDlgItem, I had used (Class*) trype of static cast. One of my team mates came back to me saying that I should be using static_cast<class*>(expression) instead of (class*) type of static cast as the latter is of c style casting. So I wanted to know that is there any specific advantages or disadvantages of using static_cast<class*> (expression) over (class*).
ciao

[Graham]

To sum up: there are two main reasons to prefer C++-style casts over C-style

1) C++ casts are safer, since you need to use the correct one. static_cast, for example, won't accidentally cast away constness; dynamic_cast returns a null pointer if the cast is not legal. C-style casts just go ahead and screw up your program.

2) C++-style casts are easier to find with a search in the editor.

[The Saint]

Thank you one and all mate.