What are different types of typecasting supported by C++

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What are different types of typecasting supported by C++..

Answer / lovly

Implicit conversion
Implicit conversions do not require any operator. They are
automatically performed when a value is copied to a
compatible type. For example:

short a=2000;
int b;
b=a;

Here, the value of a has been promoted from short to int
and we have not had to specify any type-casting operator.
This is known as a standard conversion. Standard
conversions affect fundamental data types, and allow
conversions such as the conversions between numerical types
(short to int, int to float, double to int...), to or from
bool, and some pointer conversions. Some of these
conversions may imply a loss of precision, which the
compiler can signal with a warning. This can be avoided
with an explicit conversion.

Implicit conversions also include constructor or operator
conversions, which affect classes that include specific
constructors or operator functions to perform conversions.
For example:

class A {};
class B { public: B (A a) {} };

A a;
B b=a;

Here, a implicit conversion happened between objects of
class A and class B, because B has a constructor that takes
an object of class A as parameter. Therefore implicit
conversions from A to B are allowed.

Explicit conversion
C++ is a strong-typed language. Many conversions, specially
those that imply a different interpretation of the value,
require an explicit conversion. We have already seen two
notations for explicit type conversion: functional and c-
like casting:

short a=2000;
int b;
b = (int) a; // c-like cast notation
b = int (a); // functional notation

The functionality of these explicit conversion operators is
enough for most needs with fundamental data types. However,
these operators can be applied indiscriminately on classes
and pointers to classes, which can lead to code that while
being syntactically correct can cause runtime errors. For
example, the following code is syntactically correct:

// class type-casting
#include <iostream>
using namespace std;

class CDummy {
float i,j;
};

class CAddition {
int x,y;
public:
CAddition (int a, int b) { x=a; y=b; }
int result() { return x+y;}
};

int main () {
CDummy d;
CAddition * padd;
padd = (CAddition*) &d;
cout << padd->result();
return 0;
}

The program declares a pointer to CAddition, but then it
assigns to it a reference to an object of another
incompatible type using explicit type-casting:

padd = (CAddition*) &d;

Traditional explicit type-casting allows to convert any
pointer into any other pointer type, independently of the
types they point to. The subsequent call to member result
will produce either a run-time error or a unexpected result.

In order to control these types of conversions between
classes, we have four specific casting operators:
dynamic_cast, reinterpret_cast, static_cast and const_cast.
Their format is to follow the new type enclosed between
angle-brackets (<>) and immediately after, the expression
to be converted between parentheses.

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

The traditional type-casting equivalents to these
expressions would be:

(new_type) expression
new_type (expression)

but each one with its own special characteristics:

dynamic_cast

dynamic_cast can be used only with pointers and references
to objects. Its purpose is to ensure that the result of the
type conversion is a valid complete object of the requested
class.

Therefore, dynamic_cast is always successful when we cast a
class to one of its base classes:

class CBase { };
class CDerived: public CBase { };

CBase b; CBase* pb;
CDerived d; CDerived* pd;

pb = dynamic_cast<CBase*>(&d); // ok: derived-to-base
pd = dynamic_cast<CDerived*>(&b); // wrong: base-to-
derived

The second conversion in this piece of code would produce a
compilation error since base-to-derived conversions are not
allowed with dynamic_cast unless the base class is
polymorphic.

When a class is polymorphic, dynamic_cast performs a
special checking during runtime to ensure that the
expression yields a valid complete object of the requested
class:

// dynamic_cast
#include <iostream>
#include <exception>
using namespace std;

class CBase { virtual void dummy() {} };
class CDerived: public CBase { int a; };

int main () {
try {
CBase * pba = new CDerived;
CBase * pbb = new CBase;
CDerived * pd;

pd = dynamic_cast<CDerived*>(pba);
if (pd==0) cout << "Null pointer on first type-cast" <<
endl;

pd = dynamic_cast<CDerived*>(pbb);
if (pd==0) cout << "Null pointer on second type-cast"
<< endl;

} catch (exception& e) {cout << "Exception: " << e.what
();}
return 0;
}
Null pointer on second type-cast

Compatibility note: dynamic_cast requires the Run-Time Type
Information (RTTI) to keep track of dynamic types. Some
compilers support this feature as an option which is
disabled by default. This must be enabled for runtime type
checking using dynamic_cast to work properly.

The code tries to perform two dynamic casts from pointer
objects of type CBase* (pba and pbb) to a pointer object of
type CDerived*, but only the first one is successful.
Notice their respective initializations:

CBase * pba = new CDerived;
CBase * pbb = new CBase;

Even though both are pointers of type CBase*, pba points to
an object of type CDerived, while pbb points to an object
of type CBase. Thus, when their respective type-castings
are performed using dynamic_cast, pba is pointing to a full
object of class CDerived, whereas pbb is pointing to an
object of class CBase, which is an incomplete object of
class CDerived.

When dynamic_cast cannot cast a pointer because it is not a
complete object of the required class -as in the second
conversion in the previous example- it returns a null
pointer to indicate the failure. If dynamic_cast is used to
convert to a reference type and the conversion is not
possible, an exception of type bad_cast is thrown instead.

dynamic_cast can also cast null pointers even between
pointers to unrelated classes, and can also cast pointers
of any type to void pointers (void*).

static_cast
static_cast can perform conversions between pointers to
related classes, not only from the derived class to its
base, but also from a base class to its derived. This
ensures that at least the classes are compatible if the
proper object is converted, but no safety check is
performed during runtime to check if the object being
converted is in fact a full object of the destination type.
Therefore, it is up to the programmer to ensure that the
conversion is safe. On the other side, the overhead of the
type-safety checks of dynamic_cast is avoided.

class CBase {};
class CDerived: public CBase {};
CBase * a = new CBase;
CDerived * b = static_cast<CDerived*>(a);

This would be valid, although b would point to an
incomplete object of the class and could lead to runtime
errors if dereferenced.

static_cast can also be used to perform any other non-
pointer conversion that could also be performed implicitly,
like for example standard conversion between fundamental
types:

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

Or any conversion between classes with explicit
constructors or operator functions as described
in "implicit conversions" above.

reinterpret_cast
reinterpret_cast converts any pointer type to any other
pointer type, even of unrelated classes. The operation
result is a simple binary copy of the value from one
pointer to the other. All pointer conversions are allowed:
neither the content pointed nor the pointer type itself is
checked.

It can also cast pointers to or from integer types. The
format in which this integer value represents a pointer is
platform-specific. The only guarantee is that a pointer
cast to an integer type large enough to fully contain it,
is granted to be able to be cast back to a valid pointer.

The conversions that can be performed by reinterpret_cast
but not by static_cast have no specific uses in C++ are low-
level operations, whose interpretation results in code
which is generally system-specific, and thus non-portable.
For example:

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

This is valid C++ code, although it does not make much
sense, since now we have a pointer that points to an object
of an incompatible class, and thus dereferencing it is
unsafe.

const_cast
This type of casting manipulates the constness of an
object, either to be set or to be removed. For example, in
order to pass a const argument to a function that expects a
non-constant parameter:

// const_cast
#include <iostream>
using namespace std;

void print (char * str)
{
cout << str << endl;
}

int main () {
const char * c = "sample text";
print ( const_cast<char *> (c) );
return 0;
}
sample text

typeid
typeid allows to check the type of an expression:

typeid (expression)

This operator returns a reference to a constant object of
type type_info that is defined in the standard header file
<typeinfo>. This returned value can be compared with
another one using operators == and != or can serve to
obtain a null-terminated character sequence representing
the data type or class name by using its name() member.

// typeid
#include <iostream>
#include <typeinfo>
using namespace std;

int main () {
int * a,b;
a=0; b=0;
if (typeid(a) != typeid(b))
{
cout << "a and b are of different types:\n";
cout << "a is: " << typeid(a).name() << '\n';
cout << "b is: " << typeid(b).name() << '\n';
}
return 0;
}
a and b are of different types:
a is: int *
b is: int

When typeid is applied to classes typeid uses the RTTI to
keep track of the type of dynamic objects. When typeid is
applied to an expression whose type is a polymorphic class,
the result is the type of the most derived complete object:

// typeid, polymorphic class
#include <iostream>
#include <typeinfo>
#include <exception>
using namespace std;

class CBase { virtual void f(){} };
class CDerived : public CBase {};

int main () {
try {
CBase* a = new CBase;
CBase* b = new CDerived;
cout << "a is: " << typeid(a).name() << '\n';
cout << "b is: " << typeid(b).name() << '\n';
cout << "*a is: " << typeid(*a).name() << '\n';
cout << "*b is: " << typeid(*b).name() << '\n';
} catch (exception& e) { cout << "Exception: " << e.what
() << endl; }
return 0;
}
a is: class CBase *
b is: class CBase *
*a is: class CBase
*b is: class CDerived