C cast to derived class

Question

How can i cast to a derived class  The below approaches all give the following error      Cannot convert from BaseType to DerivedType  No constructor could take   the source type  or constructor overload resolution was ambiguous    BaseType m baseType   DerivedType m derivedType   m baseType     gives same error  DerivedType m derivedType    DerivedType m baseType     gives same error  DerivedType   m derivedType    DerivedType    amp  m baseType     gives same error

User · Answer

First of all - prerequisite for downcast is that object you are casting is of the type you are casting to. Casting with dynamic_cast will check this condition in runtime (provided that casted object has some virtual functions) and throw bad_cast or return NULL pointer on failure. Compile-time casts will not check anything and will just lead tu undefined behaviour if this prerequisite does not hold.
Now analyzing your code:

DerivedType m_derivedType = m_baseType;

Here there is no casting. You are creating a new object of type DerivedType and try to initialize it with value of m_baseType variable.

Next line is not much better:

DerivedType m_derivedType = (DerivedType)m_baseType;

Here you are creating a temporary of DerivedType type initialized with m_baseType value.

The last line

DerivedType * m_derivedType = (DerivedType*) & m_baseType;

should compile provided that BaseType is a direct or indirect public base class of DerivedType. It has two flaws anyway:

You use deprecated C-style cast. The proper way for such casts is
static_cast<DerivedType *>(&m_baseType)
The actual type of casted object is not of DerivedType (as it was defined as BaseType m_baseType; so any use of m_derivedType pointer will result in undefined behaviour.

User · Answer

Think like this   class Animal      Some virtual members       class Dog  public Animal     class Cat  public Animal       Dog     dog  Cat     cat  Animal amp  AnimalRef1   dog      Notice no cast required   Dogs and cats are animals   Animal amp  AnimalRef2   cat  Animal  AnimalPtr1    amp dog  Animal  AnimlaPtr2    amp cat   Cat amp     catRef1   dynamic cast lt Cat amp  gt  AnimalRef1       Throws an exception  AnimalRef1 is a dog Cat     catPtr1   dynamic cast lt Cat  gt  AnimalPtr1       Returns NULL         AnimalPtr1 is a dog Cat amp     catRef2   dynamic cast lt Cat amp  gt  AnimalRef2       Works Cat     catPtr2   dynamic cast lt Cat  gt  AnimalPtr2       Works     This on the other hand makes no sense    An animal object is not a cat  Therefore it can not be treated like a Cat  Animal  a  Cat amp     catRef1   dynamic cast lt Cat amp  gt  a         Throws an exception  Its not a CAT Cat     catPtr1   dynamic cast lt Cat  gt   amp a        Returns NULL         Its not a CAT    Now looking back at your first statement   Animal   animal   cat        This works  But it slices the cat part out and just                              assigns the animal part of the object  Cat      bigCat   animal     Makes no sense                               An animal is not a cat      Dog      bigDog   bigCat     A cat is not a dog        You should very rarely ever need to use dynamic cast  This is why we have virtual methods   void makeNoise Animal amp  animal         animal DoNoiseMake       Dog    dog  Cat    cat  Duck   duck  Chicken chicken   makeNoise dog   makeNoise cat   makeNoise duck   makeNoise chicken     The only reason I can think of is if you stored your object in a base class container   std  vector lt Animal  gt   barnYard  barnYard push back  amp dog   barnYard push back  amp cat   barnYard push back  amp duck   barnYard push back  amp chicken    Dog   dog   dynamic cast lt Dog  gt  barnYard 1       Note  NULL as this was the cat    But if you need to cast particular objects back to Dogs then there is a fundamental problem in your design  You should be accessing properties via the virtual methods   barnYard 1 - gt DoNoiseMake

User · Answer

dynamic cast should be what you are looking for   EDIT   DerivedType m derivedType   m baseType     gives same error   The above appears to be trying to invoke the assignment operator  which is probably not defined on type DerivedType and accepting a type of BaseType   DerivedType   m derivedType    DerivedType    amp  m baseType     gives same error   You are on the right path here but the usage of the dynamic cast will attempt to safely cast to the supplied type and if it fails  a NULL will be returned   Going on memory here  try this  but note the cast will return NULL as you are casting from a base type to a derived type    DerivedType   m derivedType   dynamic cast lt DerivedType  gt   amp m baseType     If m baseType was a pointer and actually pointed to a type of DerivedType  then the dynamic cast should work   Hope this helps

User · Answer

You can t cast a base object to a derived type - it isn t of that type   If you have a base type pointer to a derived object  then you can cast that pointer around using dynamic cast  For instance   DerivedType D  BaseType B   BaseType  B ptr  amp B BaseType  D ptr  amp D    get a base pointer to derived type  DerivedType  derived ptr1 dynamic cast lt DerivedType  gt  D ptr     works fine DerivedType  derived ptr2 dynamic cast lt DerivedType  gt  B ptr     returns NULL

[c++] C++ cast to derived class

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