Asked  7 Months ago    Answers:  5   Viewed   32 times

I have been reading through the C++ FAQ and was curious about the friend declaration. I personally have never used it, however I am interested in exploring the language.

What is a good example of using friend?


Reading the FAQ a bit longer I like the idea of the << >> operator overloading and adding as a friend of those classes. However I am not sure how this doesn't break encapsulation. When can these exceptions stay within the strictness that is OOP?

 Answers

91

Firstly (IMO) don't listen to people who say friend is not useful. It IS useful. In many situations you will have objects with data or functionality that are not intended to be publicly available. This is particularly true of large codebases with many authors who may only be superficially familiar with different areas.

There ARE alternatives to the friend specifier, but often they are cumbersome (cpp-level concrete classes/masked typedefs) or not foolproof (comments or function name conventions).

Onto the answer;

The friend specifier allows the designated class access to protected data or functionality within the class making the friend statement. For example in the below code anyone may ask a child for their name, but only the mother and the child may change the name.

You can take this simple example further by considering a more complex class such as a Window. Quite likely a Window will have many function/data elements that should not be publicly accessible, but ARE needed by a related class such as a WindowManager.

class Child
{
//Mother class members can access the private parts of class Child.
friend class Mother;

public:

  string name( void );

protected:

  void setName( string newName );
};
Tuesday, June 1, 2021
 
Rudie
answered 7 Months ago
62
char *some_memory = "Hello World";

is creating a pointer to a string constant. That means the string "Hello World" will be somewhere in the read-only part of the memory and you just have a pointer to it. You can use the string as read-only. You cannot make changes to it. Example:

some_memory[0] = 'h';

Is asking for trouble.

On the other hand

some_memory = (char *)malloc(size_to_allocate);

is allocating a char array ( a variable) and some_memory points to that allocated memory. Now this array is both read and write. You can now do:

some_memory[0] = 'h';

and the array contents change to "hello World"

Wednesday, June 2, 2021
 
Deyson
answered 6 Months ago
63

Template Metaprogramming

typedef is necessary for many template metaprogramming tasks -- whenever a class is treated as a "compile-time type function", a typedef is used as a "compile-time type value" to obtain the resulting type. E.g. consider a simple metafunction for converting a pointer type to its base type:

template<typename T>
struct strip_pointer_from;

template<typename T>
struct strip_pointer_from<T*> {   // Partial specialisation for pointer types
    typedef T type;
};

Example: the type expression strip_pointer_from<double*>::type evaluates to double. Note that template metaprogramming is not commonly used outside of library development.

Simplifying Function Pointer Types

typedef is helpful for giving a short, sharp alias to complicated function pointer types:

typedef int (*my_callback_function_type)(int, double, std::string);

void RegisterCallback(my_callback_function_type fn) {
    ...
}
Saturday, June 12, 2021
 
Corne
answered 6 Months ago
19

More generally encapsulation refers simply to bundling the data (e.g. of an object) with the operations on that data. So you have a class encapsulating data - fields - along with the methods for manipulating that data.

But encapsulation is also sometimes used in the same way as your answer, and indeed, one of the points of bundling data and methods is to hide the implementation.

I suppose a better answer than just use methods and make all fields private is: use interfaces. This way, operations on an object are purely based on the interface contract, and are in no way tied to the fields or helper methods used to implement that contract internally.

Friday, July 9, 2021
 
nasty
answered 5 Months ago
55

Well I know that making fields private and then making setter and getter of the fields is encapsulation. However, does encapsulation mean just this?

---> Encapsulation is an OOP concept where object state(class fields) and it's behaviour(methods) is wrapped together. Java provides encapsulation using class.

Information Hiding:

--> mechanism for restricting access to some of the object's components. Your above example is the case of Information Hiding if you make age private.


Initially, Information/Data Hiding was considered the part of Encapsulation, and the definitions of Encapsulation would be as:

  • A language mechanism for restricting access to some of the object's components.
  • A language construct that facilitates the bundling of data with the methods (or other functions) operating on that data.

the second definition is motivated by the fact that in many OOP languages hiding of components is not automatic or can be overridden; thus, information hiding is defined as a separate notion by those who prefer the second definition.

Reference: wikipage

Wednesday, July 28, 2021
 
the_e
answered 4 Months ago
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