Pointers to classes (From the note of my firend)



Pointers to classes

Objects can also be pointed to by pointers: Once declared, a class becomes a valid type, so it can be used as the type pointed to by a pointer. For example:

Rectangle * prect;

is a pointer to an object of class Rectangle.

Similarly as with plain data structures, the members of an object can be accessed directly from a pointer by using the arrow operator (->). Here is an example with some possible combinations:

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// pointer to classes example #include <iostream> using namespace std; class Rectangle { int width, height; public: Rectangle(int x, int y) : width(x), height(y) {} int area(void) { return width * height; } }; int main() { Rectangle obj (3, 4); Rectangle * foo, * bar, * baz; foo = &obj; bar = new Rectangle (5, 6); baz = new Rectangle[2] { {2,5}, {3,6} }; cout << "obj's area: " << obj.area() << ' '; cout << "*foo's area: " << foo->area() << ' '; cout << "*bar's area: " << bar->area() << ' '; cout << "baz[0]'s area:" << baz[0].area() << ' '; cout << "baz[1]'s area:" << baz[1].area() << ' '; delete bar; delete[] baz; return 0; }

This example makes use of several operators to operate on objects and pointers (operators *, &, ., ->, []). They can be interpreted as:

expression	can be read as
*x	pointed to by x
&x	address of x
x.y	member y of object x
x->y	member y of object pointed to by x
(*x).y	member y of object pointed to by x (equivalent to the previous one)
x[0]	first object pointed to by x
x[1]	second object pointed to by x
x[n]	(n+1)th object pointed to by x

Most of these expressions have been introduced in earlier chapters. Most notably, the chapter about arrays introduced the offset operator ([]) and the chapter about plain data structures introduced the arrow operator (->).

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