Static Cast

The expression

static_cast<T>(v) converts the value of the expression

v to that of type

T. It can be used for any cast that is performed implicitly on assignment. In addition, any value may be cast to

void, and any implicit cast can be reversed if that cast would be legal as an old-style cast. It cannot be used to cast away

const.

class B { public: virtual void g( ); }; class C : public B { public: virtual void g( ); }; void use_of_static_cast( ) { C c; // an explicit temporary lvalue to the base of c, a B B& br = c; br.g( ); // call B::g instead of C::g // a static_cast of an lvalue to the base of c, a B static_cast (c).g( ); } // call B::g instead of C::g

Dynamic Cast

A pointer or reference to a class can actually point to any class publicly derived from that class. Occasionally, it may be desirable to obtain a pointer to the fully-derived class, or to some other base class for the object. The dynamic cast provides this facility.

The dynamic type cast will convert a pointer or reference to one class into a pointer or reference to another class. That second class must be the fully-derived class of the object, or a base class of the fully-derived class.

In the expression

dynamic_cast<T>(v),

v is the expression to be cast, and

T is the type to which it should be cast.

T must be a pointer or reference to a complete class type, or "pointer to

cv

void", where

cv is [

const][

volatile]. In the case of pointer types, if the specified class is not a base of the fully

derived class, the cast returns a null pointer. In the case of reference types, if the specified class is not a base of the fully derived class, the cast throws a

bad_cast exception. For example, given the class definitions:

class A { public: virtual void f( ); }; class B { public: virtual void g( ); }; class AB : public virtual A, private B { };

The following function will succeed.

void simple_dynamic_casts( ) { AB ab; B* bp = (B*)&ab; // cast needed to break protection A* ap = &ab; // public derivation, no cast needed AB& abr = dynamic_cast (*bp); // succeeds ap = dynamic_cast (bp); assert( ap != NULL ); bp = dynamic_cast (ap); assert( bp == NULL ); ap = dynamic_cast (&abr); assert( ap != NULL ); bp = dynamic_cast (&abr); assert( bp == NULL ); }

In the presence of virtual inheritance and multiple inheritance of a single base class, the actual dynamic cast must be able to identify a unique match. If the match is not unique, the cast fails. For example, given the additional class definitions:

class AB_B : public AB, public B { }; class AB_B__AB : public AB_B, public AB { };

The following function will succeed:

void complex_dynamic_casts( ) { AB_B__AB ab_b__ab; A*ap = &ab_b__ab; // okay: finds unique A statically AB*abp = dynamic_cast (ap); // fails: ambiguous assert( abp == NULL ); // STATIC ERROR: AB_B* ab_bp = (AB_B*)ap; // not a dynamic cast AB_B*ab_bp = dynamic_cast (ap); // dynamic one is okay assert( ab_bp != NULL ); }

The null-pointer error return of

dynamic_cast is useful as a condition between two bodies of code, one to handle the cast if the type guess is correct, and one if it is not.

void using_dynamic_cast( A* ap ) { if ( AB *abp = dynamic_cast (ap) ) { // abp is non-null, // so ap was a pointer to an AB object // go ahead and use abp process_AB( abp ); } else { // abp is null, // so ap was NOT a pointer to an AB object // do not use abp process_not_AB( ap ); }

If run-time type information has been disabled, i.e.

-features=no%rtti, (See Chapter 5, "RTTI"), the compiler converts

dynamic_cast to

static_cast and issues a warning.

If exceptions have been disabled (See Chapter 4, "Exception Handling"), the compiler converts

dynamic_cast<T&> to

static_cast<T&> and issues a warning. The dynamic cast to a reference may require an exception in normal circumstances.

Dynamic cast is necessarily slower than an appropriate design pattern, such as conversion by virtual functions. See

Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma et al.