Why class is needed instead of structures

Question

Can some one please tell me why exactly classes were introduced in C++. Are there some things that classes can perform that cannot be achieved using structures?

Answer 1

Read Stroustrup's explanation here: What is so great about classes?

A class is the representation of an idea, a concept, in the code. An object of a class represents a particular example of the idea in the code. Without classes, a reader of the code would have to guess about the relationships among data items and functions - classes make such relationships explicit and "understood" by compilers. With classes, more of the high-level structure of your program is reflected in the code, not just in the comments.

A well-designed class presents a clean and simple interface to its users, hiding its representation and saving its users from having to know about that representation. If the representation shouldn't be hidden - say, because users should be able to change any data member any way they like - you can think of that class as "just a plain old data structure"; for example:

struct Pair {
        Pair(const string& n, const string& v) : name(n), value(v) { }
        string name, value;
};

Note that even data structures can benefit from auxiliary functions, such as constructors.

Answer 2

Actually, structures in C++ are a special case of classes, and are included to make C++ more backward compatible with C. Recall from C that a structure is a description of a collection of data items of potentially different types that's given a name. This is also somethimes called a record in other languages.

In C++ there is a basic thing called a class, wich is a collection of data items of potentially different types along with functions that operate on them.

In C, a structure lets you define a chunk of memory using the structure declaration as a pattern.

In C++, a class lets you define a chunk of memory using the class declaration as a pattern, and have it associated with functions that operate on it.

In C, a common pattern to do things with a structure is something like this:

struct T {
   ?* some declarations */
};

struct T * foo.

struct T * create_a_T(}{/* malloc memory */}
void init_a_T(struct T * t){/* set it up */}
void do_something_with_T(struct T * t, ?* more arguments*/){/* more good things */}

In C++, a class method works exactly the same way, except that you don't need to have the struct T * as an argument; that's take care of automatically.

Along with all that, in C++ a class lets you make certain elements of the class "private" or "protected", which is to say they're not accessible from outside the class methods. This can protect you from many kinds of programming errors.

Also, in C++ you can use inheritance, which means that you can define a new class that is "just like" an existing class, except for code that adds other data declarations or methods. This lets you organize the code around a relationship called "a kind of", so, for example, you can have a class Car, and define a new class Mercedes that is "a kind of" Car, but has special Mercedes-related pieces.

All of this goes along with the notion of "object oriented programming" which is too much to explain in one answer, but which has historically proven to make it easier to build large programs correctly.

Answer 3

class CL
{
    int x;
};

struct ST
{
    int x;
};

The only difference that ST and CL have is that CL.x is private and ST.x is public.

Best practice: when you define a small data structure with only public objects use the keyword struct, differently use the keyword class.

Update:

this is valid --> class A;

this is invalid --> struct B; // error: forward declaration of struct B

Answer 4

C++ is oriented to objects, and the objets are made by classes, so C++ only use it