17.-C-MODULE-06

Summary

Cast	Conversion	Reinterpretation	Upcast	Downcast	Type qualifier
Implicit	Yes		Yes
Static	Yes		Yes	Yes
Dynamic_Cast			Yes	Yes
Const_cast					Yes
Reinterpret_cast		Yes	Yes	Yes
Legacy C Cast	Yes	Yes	Yes	Yes	Yes

Casts

1. 5 Types of casts

• Conversion types
• Reinterpretation types
• Qualifier reinterpretation types
• Downcast / Upcast

2. Cast C++

• Static cast
• Dynamic cast
• Reinterpret cast
• Const cast

3. Operators types & Explicit

• Type cast operator
• Explicit

1. 5 Conversion types

Conversion = Changement de type = Reorganisation des bits pour garder la même valeur. Ch

int    a;
double b = a;                 // Implicit conversion cast  + implicit promotion
double c = (double) a;        // Explicit conversion cast
int    d = (int) n;           // Implicit demotion

Reinterpretation types

Reinterpretation = conversion identitaire = Changement de type d'adresse = le codage des bits n'est pas modifié void * est le type le plus générale, moins précis

float a;
void   b  = &a;               // Implicit reinterpretation cast
void * b  = (void *) a;       // Implicit reinterpretation cast

void * d  = &a;               // Implicit promotion 
int  * e  = d;                // Implicit demotion    -> Bad ! 
int  * f   = (int *) d;       // Explicit demotion

Type qualifier reinterpretation (example = const)

The reinterpretation needs to be explicit otherwise does not compile.

int const * d = 1;
int       * e = d;            // Implicit demotion -> WARNING, doesn't compile
int       * e = (int *)d;     // Explicit demotion 

Downcast / Upcast

The reinterpretation needs to be explicit otherwise does not compile. Une classe parent est plus générale que la classe enfant.

2. C++ casts

Static cast

En C++, on utilise static_cast pour downcast, upcast and conversion dans la même hierarchie de classes.

int         a = 42;
double      b = a;                       // Implicit promotion 
int       * e = static_cast<int>(b);     // Explicit demotion 

Dynamic cast

Le cast se fait à l'execution. De plus il faut qu'il y ait une méthode dans la classe. (Polymorphisme par sous-typage)

class Parent {public : virtual ~Parent(void) {}};
class Child1 : public Parent {};
class Child2 : public Parent {};

Child1       a;
Parent      *b = &a;                       // Implicit upcast
try
{
  Child1      *c = dynamic_cast<Child1 *>(b);
  Child2     & d = dynamic_cast<Child2 &>(*b);    -> error
}
catch (std::bad_cast &bc)
{
  cout << "Cast error" << endl;
}

Reinterpret cast

Le cast le plus permissif. Il convertit anyway.

float     a = 420.042f;

void    * b = & a;
int     * c = reinterpret_cast<int *> (b);  -> ok
int     & d = reinterpret_cast<int &> (b);  -> ok

Const cast (should not be used)

Le cast entre type constant et type mutable. = cast de qualifieur de type

float     a = 420.042f;

void    * b = & a;
int     * c = reinterpret_cast<int *> (b);  -> ok
int     & d = const_cast<int &> (b);  -> ok

3. Operators & Explicit

Type cast operator

Allow to implicit cast thanks to the class.

class foo {
  public :
      operator float() {return _value ;}
      operator int()   {return static_cast<int>(_value) ;}
  private :
      float _value;}
     
    foo   a(420.024f);
    float b = a;
    int   c = a;     

Explicit

No conversion is allowed in the constructor.