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const_cast<target-type> (expr) dynamic_cast<target-type> (expr) reinterpret_cast<target-type> (expr) static_cast<target-type> (expr) Here, target-type specifies the target type of the cast and expr is the expression being cast to the new type The const_cast operator is used to explicitly override const and/or volatile in a cast The target type must be the same as the source type except for the alteration of its const or volatile attributes The most common use of const_cast is to remove const-ness dynamic_cast performs a run-time cast that verifies the validity of the cast If the cast cannot be made, the cast fails and the expression evaluates to null Its main use is for performing casts on polymorphic types For example, given two polymorphic classes B and D, with D derived from B, a dynamic_cast can always cast a D* pointer into a B* pointer A dynamic_cast can cast a B* pointer into a D* pointer only if the object being pointed to actually is a D* In general, dynamic_cast will succeed if the attempted polymorphic cast is permitted (that is, if the target type can legally apply to the type of object being cast) If the cast cannot be made, then dynamic_cast evaluates to null The static_cast operator performs a nonpolymorphic cast For example, it can be used to cast a base class pointer into a derived class pointer It can also be used for any standard conversion No run-time checks are performed The reinterpret_cast operator changes one type into a fundamentally different type For example, it can be used to change a pointer into an integer A reinterpret_cast should be used for casting inherently incompatible pointer types Only const_cast can cast away const-ness That is, neither dynamic_cast, static_cast, nor reinterpret_cast can alter the const-ness of an object The following program demonstrates the use of dynamic_cast

#include <iostream> using namespace std; #define NUM_EMPLOYEES 4 class employee { public: employee() { cout << "Constructing employee\n"; } virtual void print() = 0; }; class programmer : public employee { public: programmer() { cout << "Constructing programmer\n"; }

Also note that the behaviors of desktop-related actions within an MDI child form are modified. For example, the Minimize and Maximize buttons on the title bar work within the parent window, rather than on the desktop itself. In the rest of this chapter we will enhance our MyPhotos application to support a multiple document interface. We begin with the MDI container form.

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void print() { cout << "Printing programmer object\n"; } }; class salesperson : public employee { public: salesperson() { cout << "Constructing salesperson\n"; } void print() { cout << "Printing salesperson object\n"; } }; class executive : public employee { public: executive() { cout << "Constructing executive\n"; } void print() { cout << "Printing executive object\n"; } }; int main() { programmer prog1, prog2; executive ex; salesperson sp; // Initialize the array of employees employee *e[NUM_EMPLOYEES]; e[0] = &prog1; e[1] = &sp; e[2] = &ex; e[3] = &prog2; // See which ones are programmers for(int i = 0; i < NUM_EMPLOYEES; i++) { programmer *pp = dynamic_cast<programmer*>(e[i]); if(pp) { cout << "Is a programmer\n"; pp->print(); } else { cout << "Not a programmer\n"; } } }

So let s convert our existing MyPhotos application into an MDI application. This initial work is not as difficult as you might think. Generally, we need one Form to act as the top-level container, and the ability to create other forms as children within this container. Here, we will do this via the following tasks:

The array e contains pointers to the four employees The dynamic_cast operator is used to identify which of these are programmers If the dynamic_cast operator returns a

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Create a new parent form for the application to act as the MDI container. Add a menu bar and New menu item to create MDI child forms. Define a new Main method in the parent as the entry point for the application.