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It is possible for a nonmember function to have access to the private members of a class by declaring it as a friend of the class For example, here frd( ) is declared to be a friend of the class cl:
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class cl { // public: friend void frd(); // }; C++
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As you can see, the keyword friend precedes the entire function declaration One reason that friend functions are allowed in C++ is to accommodate situations in which, for the sake of efficiency, two classes must share the same function To see an example, consider a program that defines two classes called line and box The class line contains all necessary data and code to draw a horizontal dashed line of any specified length, beginning at a specified x,y coordinate using a specified color The box class contains all code and data to draw a box at the specified upper-left and lower-right coordinates in a specified color Both classes use the same_color( ) function to determine whether a line and a box are drawn in the same color These classes are declared as shown here:
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class line; class box { int color; // color of box int upx, upy; // upper left corner int lowx, lowy; // lower right corner public: friend int same_color(line l, box b); void set_color(int c); void define_box(int x1, int y1, int x2, int y2); void show_box();
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} ; class line { int color; // color of line int startx, starty; // coordinates int len; // length public: friend int same_color(line l, box b); void set_color(int c); void define_line(int x, int y, int l); void show_line(); } ;
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The same_color( ) function, which is a member of neither class but a friend of both, returns true if both the line object and the box object, which form its arguments, are drawn in the same color; it returns 0 otherwise The same_color( ) function is defined as:
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// Return true if line and box have same color int same_color(line l, box b) { if(lcolor==bcolor) return 1; return 0; }
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As you can see, the same_color( ) function needs access to the private members of both line and box to perform its task efficiently Being a friend of each class grants it this access privilege Further, notice that because same_color( ) is not a member, no scope resolution operator or class name is used in its definition (Remember that public functions can be created to return the colors of both line and box, and any function could have compared their colors However, such an approach requires extra function calls, which in some cases is inefficient) Notice the empty declaration of line at the start of the class declarations Since same_color( ) in box refers to line before line is declared, line must be forward referenced If this is not done, the compiler will not know about line when it is encountered in the declaration of box In C++, a forward reference to a class is simply the keyword class followed by the type name of the class Here is a program that demonstrates the line and box classes and illustrates how a friend function can access the private members of a class (This program must be run in a console session under Windows)
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21:
A Closer Look at Classes and Objects
#include <iostream> #include <conioh> using namespace std; class line; class box { int color; // color of box int upx, upy; // upper left corner int lowx, lowy; // lower right corner public: friend int same_color(line l, box b); void set_color(int c); void define_box(int x1, int y1, int x2, int y2); void show_box(); } ; class line { int color; // color of line int startx, starty; // coordinates int len; // length public: friend int same_color(line l, box b); void set_color(int c); void define_line(int x, int y, int l); void show_line(); } ; // Return true if line and box have same color int same_color(line l, box b) { if(lcolor==bcolor) return 1; return 0; } void box::set_color(int c) { color = c; } void line::set_color(int c)
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