FAQ 27.11 What is a downcast?

A downcast is the conversion of a Base* to a Derived*, where class Derived is publicly derived from class Base. A downcast is used when the client code thinks (or hopes!) that a Base* points to an object of class Derived or a class derived from Derived and it needs to access a member function that is provided by Derived but not by Base.

For example, suppose class LiquidAsset is derived from class Asset, and LiquidAsset is a derived class that is liquidatable but Asset itself is not liquidatable. A downcast from an Asset* to a LiquidAsset* allows the liquidation.

 #include <iostream> using namespace std; class Asset { public:   virtual ~Asset() throw();   virtual bool isLiquidatable() const throw(); }; Asset::~Asset() throw() { } bool Asset::isLiquidatable() const throw() { return false; } class LiquidAsset : public Asset { public:   LiquidAsset(int value=100) throw();   int  getValue() const throw();   void setValue(int value) throw();   virtual bool isLiquidatable() const throw(); protected:   int value_;    //value of this asset }; LiquidAsset::LiquidAsset(int value) throw()  : value_(value) { } int LiquidAsset::getValue() const throw()        { return value_;  } void LiquidAsset::setValue(int value) throw()    { value_ = value; } bool LiquidAsset::isLiquidatable() const throw() { return true;    } int tryToLiquidate(Asset& asset) throw() {   int value;   if (asset.isLiquidatable()) {     LiquidAsset& liquidAsset = (LiquidAsset&) asset;     value = liquidAsset.getValue();     liquidAsset.setValue(0);     cout << "Liquidated $" << value << '\n';   } else {     value = 0;     cout << "Sorry, couldn't liquidate this asset\n";   }   return value; } int main() {   Asset       a;   LiquidAsset b;   tryToLiquidate(a);   tryToLiquidate(b); } 

The output of this program follows.

 Sorry, couldn't liquidate this asset Liquidated $100 

Although dynamic_cast (see FAQ 27.17) can eliminate the unsafe casts, it cannot eliminate the nonextensible control flow logic. See FAQ 27.12 for a better alternative.