How to Use Friend Functions in C++
C++ offers three levels of data access control inside a class. Private data isn't accessible by non-member functions or outside classes. But sometimes you need to access this data in a program, so you need to find a way to work around this C++ rule. The friend feature lets a programmer access private data. Read on to learn how to use friend functions in C++.
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Instructions
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1
Find a class that has private members. Remember that data that you don't declare under an access modifier is private by default:
class Tutorial {
int privateData; // this int is private and is insulated from the outside world
public:
Tutorial(): privateData(5); // default constructor initializes privateData to 5
// ... -
2
Allow a non-member function to read privateData in Step 1. A non-member function is any function that exists outside class Tutorial. Precede the declaration of that function by the friend keyword and insert the line in the private area of Tutorial:
class Tutorial {
int privateData;
friend void Display(); // our non-member function
// ...
The private keyword tells class Tutorial that it can trust Display(), even though it is not one of its member functions. -
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3
Let another class access the private data of class Tutorial. The declaration of a friend class is similar. Precede the class declaration by the friend keyword and insert the line in the private data area of Tutorial:
class Tutorial {
int privateData;
friend void Display();
friend class Outside; // our external class
// ...
Now class Outside has access privileges to private data. -
4
Study a C++ program that demonstrates the friend concepts, as in the following code:
void Display() {
Tutorial t;
cout << t.privateData << endl;
}
class Outside {
public:
void Display() {
Tutorial x;
cout << x.Display() << endl;
};
main() {
Display();
Outside x;
x.Display();
} -
5
See the result:
5
5
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1
Tips & Warnings
Use friend with good judgment. Bjarne Stoustrup (the designer of C++) created the private access modifier for a good reason: encapsulation and data security. In other words, don't use "friend" as a hacker.
