cast.md

Cast

Type Cast

• Static Cast

// Used to explicitly find out if cast
a = f;
a = static_cast<int>(f);

// Conversion construtor
// Conversion operator
class Int {
    int x;
public:
    Int(int x=0) {
 cout << "conversion constructor" << endl;
 }
    operator string ( ) {
 cout << "conversion operator" << endl;
        return to_string(x);
 }
}

int main() {
 Int obj(3);

 // Compile time casting 
 string str = obj;
 obj = 20;
}


// Prevents char to int type cast at compile time
// In every case provides better security than c-style (void *) cast

// For compatible type conversion, such as float to int.
// For conversion operators and conversion constructors.
// To avoid unrelated pointer conversion.
// Avoids derived to private base pointer conversion.
// Use for all up-cast but never use for confused down-cast because there are no runtime checks performed
// Intensions are more clear in C++ style cast (express your intent better and make code review easier)
// Finding is easy.
//  Error found at compile-time.

• Const Cast

  • To remove const, volatile
  • When a third-party library is taking a non-const param and not changing it

• Dynamic Cast

  • To use it, must have virtual function in the base class.
  • Work only on polymorphic base class (at least one virtual function in base class) because it uses this information in runtime to decide about wrong down-cast
  • It is used for up-cast (D to B) and down-cast (B to D), but it is mainly used for correct downcast
  • Using this cast has run time overhead because it checks object types at run time using RTTI (Run Time Type Information)
  • If we are sure that we will never cast to the wrong object then we should always avoid this cast and use static_cast
  • Cast from parent to correct leaf is successful