C++ : Type Casting

Converting an expression of a given type into another type is known as type-casting. In other words, typecasting is used to convert the type of a variable, function, object, expression or return value to another type.We have already seen some ways to type cast:

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 typecasting
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 run time 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.

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;
}

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;
}

Cast Operator

Any unary expression is considered a cast expression.
The compiler treats cast-expression as type type-name after a type cast has been made. Casts can be used
to convert objects of any scalar type to or from any other scalar type. Explicit type casts are constrained by
the same rules that determine the effects of implicit conversions. Additional restraints on casts may result
from the actual sizes or representation of specific types.

// expre_CastOperator.cpp

// Demonstrate cast operator

#include <iostream>

using namespace std;

int main()

{

    double x = 3.1;

    int i;

    cout << "x = " << x << endl;

    i = (int)x; // assign i the integer part of x

    cout << "i = " << i << endl;

}

// expre_CastOperator2.cpp 

#include <iostream>
using namespace std;
class Casting{
public:
    Casting(float x):m_int(5),
    m_char('A'),m_float(&x)
    {
    }
    operator float *(){
        cout<<"Float cast *\n";
        return m_float;
    }
    operator int(){
        cout<<"int cast\n";
        return m_int;
    }
    operator char(){
        cout<<"Char cast\n";
        return m_char;
    }
private:
    float *m_float;
    int m_int;
    char m_char;
};
int main()
{
    int x;
    float *y;
    char z;
    float temp = 9.4;
    Casting *cast = new Casting(temp);
    x=*cast;
    y=*cast;
    z=*cast;
    cout<<x<<","<<*y<<","<<z<<"\n";
    delete cast;
    return 0;
} 

Output: